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A  Comparative  Study 


Area  of  Acute  Vision  in  Vertebrates 


JAMES   ROLLIN   SLONAKER 


Approz'cd  as  a  Thesis  for  the  degree  of  Doctor  of  Philosophy 

in  the  department  of  Biology  at 

Clark  University 
June  20,  1896  C.  F.  HODGE 


Reprinted  from  Journal  of  Morphology,  Vol.  XIII,  No.  3 


BOSTON 
QINN     &     CONIPANY 

W^z  Sltfjetiteum  press 
1897 


A  Comparative  Study 


Area  of  Acute  Vision  in  Vertebrates 


JAMES   ROLLIN    SLONAKER 


Approved  as  a  Thesis  for  the  degree  of  Doctor  of  Philosophy 

in  the  departtnetit  of  Biology  at 

Clark  University 
June  20,  1S96  C.  F.  HODGE 


Reprinted  from  Journal  of  Morphology,  Vol.  XIII,  No.  3 


BOSTON 

QITMN     &     CO^vIPANY 

2ri)e  aitfienafum  pregg 

1897 


A    COMPARATIVE    STUDY    OF    THE    AREA    OF 
ACUTE   VISION    IN    VERTEBRATES. 

JAMES  ROLLIN  SLONAKER, 
Fellow   in   Biology,    Clark   University. 

Introduction. 

This  investigation  has  been  pursued  during  the  past  three 
years  in  the  Neurological  Laboratory  of  Clark  University  under 
the  direction  of  Dr.  C.  F.  Hodge,  to  whom  I  am  under  great 
obligations  for  his  assistance  and  encouragement.  I  am  also 
greatly  indebted  to  Clark  University  for  the  apparatus  and 
material  which  has  made  this  work  possible. 

Up  to  the  present  I  have  been  engaged  chiefly  in  a  gross 
comparison  of  the  retina  rather  than  in  its  minute  histology, 
therefore  my  aim  will  be,  first,  to  sum  up  the  results  of  others 
and  also  to  add  my  own;  second,  to  correlate  as  far  as  possible 
the  habits  of  the  animal  with  its  visual  apparatus. 

Since  there  are  so  many  investigators  who  have  written  on 
various  phases  of  the  eye,  it  will  be  impossible  to  mention  all. 
Reference,  therefore,  will  be  made  to  only  a  few  of  the  most 
important  in  the  historical  resume  and  literature  on  the  subject. 

I  have  adopted  the  nomenclature  of  the  German  investigators 
and  called  the  structure  corresponding  to  the  macula  lutea  of 
man  the  area.  According  to  the  position  of  the  area  or  fovea 
on  the  nasal  or  temporal  side  of  the  optic  nerve  entrance,  it  is 
called  area  or  fovea  nasalis  or  temporalis. 

Historical.^ 

On  the  basis  of  the  methods  of  investigation  employed,  the 
literature  may  be  divided  into  three  periods:  (i)  from  the  ear- 

1  The  literature  on  this  subject  has  been  fully  presented  by  J.  H.  Chievitz 
(Ueber  das  Vorkommen  der  Area  centralis  retinae  in  den  vier  hoheren  Wirbel- 
thierklassen,  Arch.  f.  Anat.  u.  Entwickelungsgeschichte,  1891,  Heft  4,  5,  u.  6,  pp. 
311-321),  but  as  I  have  not  found  it  anywhere  in  English,  I  will  devote  some  space 
to  it. 


446  SLONAKER.  [Vol.  XIII. 

liest  investigations  till  about  1830,  or  previous  to  the  common 
use  of  the  microscope;  (2)  from  the  use  of  the  microscope  till 
1887,  or  a  period  when  the  old  methods  of  hardening  and 
staining  were  employed,  which  made  only  the  nuclei  and  larger 
processes  visible;  and  (3)  from  1887  to  the  present  time,  or 
since  the  use  of  the  silver  chromate  and  the  methyl-blue  methods 
of  staining,  which  make  clear  not  only  the  cells,  but  the  finest 
processes  of  both  neurites  and  dendrites. 

Although  Francesco  Buzzi  (3)  is  given  the  credit  of  having 
discovered  the  yellow  spot  in  the  human  eye  in  1782,  it  was 
not  until  1791  that  the  fovea  centralis  was  noticed.  This  dis- 
covery was  made  by  the  celebrated  German  anatomist,  Sm.  Th. 
V.  Soemmerring  (2),  and  was  called  for  a  number  of  years  the 
"  Foramen  of  Soemmerring,"  he  having  considered  it  a  per- 
foration. Buzzi,  on  the  contrary,  thought  it  merely  a  thin  and 
transparent  part  of  the  retina.  Michaelis  (4)  favored  Buzzi's 
theory,  while  Reil  (5),  Meckel  (6),  and  Home  (7)  considered  it 
a  foramen. 

The  discovery  of  the  foramen  of  Soemmerring  in  man  natu- 
rally led  to  many  investigations  in  other  classes  of  vertebrates. 
Michaelis  examined  the  eyes  of  the  dog,  swine,  and  calf,  but 
found  no  trace  of  a  fovea.  Home  (7),  however,  was  more  for- 
tunate. Knowing  the  great  similarity  which  existed  in  the 
anatomy  of  man  and  the  monkey  family,  he  wisely  chose  one  of 
the  latter,  and  consequently  was  the  first  to  discover  the  fovea 
in  the  ape  in  1798.  He  considered  it  a  real  foramen  for  the 
passage  of  a  lymphatic  vessel,  and  tried  to  correlate  it  with 
such  a  vessel  in  the  optic  nerve  of  the  sheep  and  calf.  Cuvier 
(8)  confirmed  the  presence  of  the  fovea  in  the  ape  family,  but 
he  considered  it  a  thinning  of  the  retina.  This  view  gained 
ground,  but  it  was  not  firmly  established  till  1830,  when  v. 
Ammon  (9)  demonstrated  by  the  aid  of  the  microscope  that  the 
retina  was  continuous  through  the  fovea. 

Albers  (10)  found  in  1808  "a  central  hole  surrounded  by  a 
yellow  border  "  in  the  giant  tortoise  (Testudo  mydas),  but  was 
not  able  to  confirm  such  an  appearance  in  the  second  eye. 

Knox  (11)  in  1823  was  the  first  to  demonstrate  the  presence 
of  a  fovea  in  animals  other  than  the  primates.     He  examined 


No.  3-]  ACUTE    VISION  IN  VERTEBRATES.  447 

the  eyes  of  some  reptiles  and  demonstrated  the  presence  of  a 
well-defined  fovea  in  the  lizard  (Lacerta  scutata)  and  the  cha- 
meleon, Joh.  Muller(i2)  says:  "  Aforamen  centrale  is  present 
in  the  middle  point  of  the  retina  of  other  reptiles,  which  is  not 
visible  as  in  man,  where  the  limiting  membrane  is  unharmed, 
but  where  the  choroid  shows  through."  H.  Miiller  {13),  who 
so  admirably  describes  the  eye  of  the  chameleon  and  the  retina 
of  the  different  classes  of  vertebrates,  describes  a  well-defined 
fovea  found  in  many  birds,  while  in  some  birds  two  foveae  are 
present.  He  also  states  that  in  mammals  an  area  centralis  is 
present,  which  approaches  in  structure  the  yellow  spot  of  the 
human  retina. 

Three  things  were  always  sought  for  by  the  early  investiga- 
tors: the  yellow  spot,  the  foramen,  and  the  folds  of  the  retina, 
which  more  or  less  concealed  the  foramen.  Though  Home  had 
described  these  folds  as  due  to  post  mortem  changes  and  not 
present  in  the  fresh  eye,  they  were  considered  as  normal  by 
many  writers  even  as  late  as  the  middle  of  the  present  cen- 
tury (i). 

The  old  theory  that  the  fovea  was  a  foramen  which  enlarged 
and  contracted  with  the  intensity  of  the  light,  thus  protecting 
the  retina  from  injury,  rapidly  gave  place  with  the  use  of  the 
microscope  to  the  opposite  view,  that  it  was  the  place  of  acute 
vision.  The  microscope  further  brought  out  the  fact  that  the 
cells  of  the  yellow  spot  had  a  definite  arrangement,  and  that 
this  arrangement  might  be  present  without  a  fovea.  With  this 
thought  in  view  investigations  were  made  in  all  classes  of  ver- 
tebrates with  the  result  that  a  fovea  has  been  found  in  each 
class,  and  that  an  area  centralis  is  quite  common. 

Hulke  (14)  has  described  the  presence  of  a  point  in  the  retina 
of  several  amphibians  and  reptiles  which,  owing  to  a  similar 
arrangement  of  cells,  he  thinks  corresponds  to  the  human  fovea. 
Gulliver  (15)  has  described  the  presence  of  a  fovea  in  the  fish, 
and  Carriere  (16)  in  Hippocampus.  Hoffmann  (17)  describes 
an  arrangement  in  the  crocodile  which  corresponds  to  that  in 
the  fovea,  Krause  (18)  treats  of  the  eyes  of  different  verte- 
brates, and  states  that  the  dove  and  cat  possess  a  fovea,  while 
the  chicken  and  dog  do  not.     He  seems  to  be  the  only  person 


448  SLO MAKER.  [Vol.  XIII. 

who  has  found  a  fovea  in  the  cat.  Ganser  (19)  and  Chievitz 
(3 1 )  have  found  only  an  area. 

Chievitz  has  described  and  pictured  the  area  and  fovea  cen- 
tralis in  many  animals,  and  put  his  results  in  a  concise  tabu- 
lated form.  Other  investigators  of  this  period  will  be  mentioned 
later  in  a  similar  tabulation. 

Many  obscure  points  were  made  clear  by  these  numerous 
investigators.  However,  two  points  still  remained  unsolved: 
the  structure  of  the  molecular  layers  and  the  endings  of  the 
fine  branches  of  the  retinal  cells.  The  solution  of  these  points 
depended  on  a  new  method  of  research.  This  new  method 
was  inaugurated  by  Tartuferi  (21)  in  1887,  who  used  the  quick 
method  of  Golgi  and  succeeded  in  showing  that  the  cell  pro- 
cesses end  in  more  or  less  fine  tufts  which  did  not  anastomose 
with  other  bunches.  Later  on  he  discovered  and  described  the 
structure  of  the  molecular  layers, 

Dogiel  (22)  in  1888  so  modified  the  Ehrlich  method  that  it 
would  stain  the  fresh  retina.  He  was  thus  able  to  confirm 
almost  all  the  results  of  Tartuferi.  He  found  that  the  branches 
from  different  cells  anastomose,  but  other  investigators  have 
not  confirmed  this.  The  works  of  Baquis  (23)  and  Ramon  y 
Cajal  (24),  who  used  the  Golgi  method,  in  general  confirm  the 
results  previously  obtained.  Ramon  y  Cajal  has  made  clear 
the  endings  of  the  rod  and  cone  fibres  in  the  outer  molecular 
layer.  He  finds  that  there  are  certain  cells  of  the  inner  nuclear 
layer  which  are  related  to  the  cones.  That  is,  their  terminal 
branches  come  in  contact  only  with  the  processes  from  the 
cones,  while  other  cells  of  this  layer  send  their  dendrites  to  the 
rods.  In  general,  each  cell  communicates  with  many  rods  or 
cones,  excepting  in  the  fovea,  where  the  process  from  each  cell 
branches  very  little  and  comes  in  contact  with  but  one  cone. 

Methods. 

As  I  have  only  attempted  a  gross  comparison  of  the  areas  of 
acute  vision  in  this  study,  I  have  used  only  those  hardening 
fluids  and  methods  of  research  which  will  preserve  the  eye  with 
the  least  possible  distortion.     For  fine  histological  study  of  the 


No.  3-] 


ACUTE    VISION  IN  VERTEBRATES. 


449 


retinal  cells,  other  methods  such  as  that  of  Golgi  or  Ehrlich  are 
preferable. 

For  my  purposes  it  is  necessary  to  obtain  the  eye  fresh,  at 
least  not  later  than  an  hour  after  death,  and  subject  it  to  the 
action  of  certain  hardening  fluids  which  will  permeate  and  pre- 
serve without  distorting  the  eye.  Post  vioj'tetn  changes  occur 
in  the  retina  very  soon,  such  as  wrinkling  in  the  neighborhood 
of  the  fovea,  which  obscure  its  .shape  and  size  and  make  sec- 
tions through  it  of  little  value.  The  eye  is  carefully  oriented 
in  every  case  before  it  is  removed  from  the  head  by  sewing  a 
small  tag  to  the  outer  layers  of  the  sclerotic  (Fig.  i).     In  no 


Fig.  I. 

case  should  the  eye  be  punctured  in  removal,  for  this  invariably 
causes  wrinkling  of  the  retina  and  distortion  of  the  ball. 

I  have  tried  many  hardening  fluids,  but  find  that  Perenyi's 
fluid  works  the  best.  It  not  only  preserves  the  eye  with  little 
distortion,  but  also  decalcifies  all  bone,  thus  making  sections 
even  through  the  whole  head  with  eyes  in  situ  possible.  The 
different  per  cents  of  formaline  which  I  have  used  have  not 
proved  satisfactory,  as  they  caused  wrinkHng  of  the  retina. 

The  former  injection  method  ^  is  now  wholly  replaced  by  that 
of  simple  immersion,  which  is  as  follows :  after  the  eye  is  prop- 
erly tagged,  it  is  carefully  removed  and  immersed  for  from  24 
to  36  hours  in  Perenyi's  fluid.  The  time  depends  upon  the 
size  of  the  eye  and  the  amount  of  bone  to  be  decalcified.  It  is 
then  changed  to  70^  alcohol,  and  allowed  to  remain  24  hours. 
Quite  frequently  when  this  change  is  made  the  ball  caves  in 
and  becomes  somewhat  distorted.     This  may  be  prevented  or 

1  American  Naturalist,  January,  1S96,  p.  24. 


450  SLONAKER.  [Vol.  XIII. 

the  eye  again  made  perfect  by  injecting  into  the  vitreous 
chamber  with  a  hypodermic  syringe  enough  70^  alcohol  to  fill 
out  the  eye.  It  is  kept  24  hours  in  each  of  the  following 
liquids:  80,  90,  95,  loo/o  alcohol,  and  a  mixture  of  absolute 
alcohol  and  ether  (one  part  each). 

The  eye  is  now  well  hardened  and  the  front  half  may  be  cut 
off,  leaving  the  posterior  half  uninjured.  After  the  hardened 
vitreous  humor  is  removed  the  retina  is  exposed  to  view.  The 
entrance  of  the  optic  nerve,  area  and  fovea  centralis,  if  present, 
and  the  larger  blood-vessels  will  be  easily  seen.  In  many  cases 
the  area  is  very  indistinct  and  the  blood-vessels  wanting  or  so 
meagre  as  to  be  invisible  to  the  naked  eye. 

When  one  wishes  to  section  the  eye,  a  window  is  cut  in  the 
same  plane  of  the  desired  sections  and  the  hardened  vitreous 
humor  carefully  removed  without  injury  to  the  retina  or  other 
structures.  It  is  then  changed  to  celloidin.  Best  results  are 
obtained  when  three  grades  of  celloidin  are  used:  (i)  very 
dilute  ;  (2)  less  dilute  ;  (3)  as  thick  as  will  run.  It  is  allowed 
to  remain  from  four  to  six  days  in  the  first,  six  to  eight  days  in 
the  second,  and  ten  to  fifteen  days  or  longer  in  the  third.  It 
is  then  mounted  on  a  block  and  cut  in  80^0  alcohol.  In  every 
case  when  sufficient  material  was  at  hand  sections  were  made 
in  vertical  and  horizontal  planes.  Serial  sections  were  always 
saved  through  the  fovea,  so  that  the  central  section  could  be 
readily  distinguished.  Sections  were  stained  in  haematoxylin 
and  eosin  and  mounted  in  balsam. 

In  order  to  demonstrate  more  quickly  the  presence  or 
absence  of  an  area  or  fovea  centralis,  the  whole  head  of  small 


Fig.  2.: — Snow-bird  (Junco  hyemalis)  3/1. 
/.  Fovea  centralis.  .A''.  Nerve  entrance.  P.  Pecten. 


No.  3-]  ACUTE    VISION  IN  VERTEBRATES.  451 

animals  was  immersed  in  the  Perenyi  for  from  three  to  six 
hours,  when  the  anterior  part  of  the  eye  was  hardened  so  that 
the  cornea,  lens,  and  vitreous  humor  were  easily  removed, 
leaving  the  posterior  half  in  situ.  Better  results  are  obtained 
when   the  skin  is  removed  from  the  head  before  immersion. 


a    f<S  .M 


Fig.  3.  —  Tern  (Sterna  hirundo)  i/i. 

•V.  Nerve  entrance.  a.  Band-like  area.  ft.  Area  and  fovea  temporalis. 

P.  Pecten.  fn.  Area  and  fovea  nasalis. 

With  birds  I  have  had  good  results,  the  retina  lying  back 
smoothly  so  that  the  fovea  and  entrance  of  the  optic  nerve, 
marked  by  the  pecten,  may  be  easily  seen.  Figs.  2  and  3 
represent  the  appearance  of  the  retina  after  the  front  of  the 
eye  has  been  removed.  With  other  animals,  especially 
mammals,  there  is  a  greater  tendency  for  the  retina  to 
wrinkle. 

Permanent  demonstration  material  may  be  prepared  by 
subjecting  the  whole  head  to  the  different  fluids  as  described 
for  the  hardening  of  the  eye.  It  is  not  necessary,  however,  to 
carry  it  farther  than  8ofo  alcohol  when  the  front  half  of  the 
eye  and  vitreous  humor  may  be  removed.  Such  material  is 
preserved  in  8ofo  alcohol. 

Sections  were  made  through  the  whole  head  of  several 
animals  (fishes,  amphibians,  reptiles,  birds,  and  some  small 
mammals)  in  order  to  determine  approximately  the  angles 
which  the  lines  of  vision  make  with  the  median  plane.  The 
plane  of  section  passed  through  each  fovea  or  center  of  area 
centralis  and  the  center  of  the  pupil.  Fig.  4  represents  such 
a  section  through  the  foveae, /,_/",  of  a  chickadee's  head  (Parus 
atricapillus),  while  the  lines  GH  and  GI  show  the  axes  of 
vision.     The  axes  of  vision,  owing  to  the  mobility  of  the  eye, 


452 


SLONAKER. 


[Vol.  XIII. 


Fig.  4.  —  Chickadee  (Panis  atricapillus)  3/1. 
f  Fovea.  /    Pecten.  GH  and  GI  Axes  of  vision. 

may  be  greater  or  less  during  life.  The  pecten,  p,  marks  the 
entrance  of  the  optic  nerve.  When  a  second  fovea  is  present 
it  is  situated  on  the  temporal  side  of  the  nerve  entrance,  as 
shown  in  Fig.  5. 

In  order  to  show  the  relation  of  the  retinal  arteries  to  the 
area  and  fovea  centralis,  they  were  injected  with  the  gelatine- 
carmine  mass  of  Ranvier.  In  small  animals  this  injection  was 
made  in  the  carotid  arteries,  while  with  large  animals  the  eyes 
were  removed  and  the  injection  made  into  that  branch  of  the 
ophthalmic  artery  which  supplies  the  retina.  After  injection 
the  eye  was  at  once  cooled  and  hardened  in  alcohol.  When 
hardened,  the  front  half  of  the  globe  and  the  vitreous  humor 
were  carefully  removed,  exposing  to  view  the  retina,  arteries, 
entrance  of  nerve,  and  area  and  fovea  centralis,  when  present. 
Usually  the  fovea  is  readily  seen  if  it  is  present,  but  the  area 
is  sometimes  very  difficult  to  discern,  and  were  it  not  for  the 
blood-vessels  acting  as  landmarks,  it  might  be  overlooked 
altogether.  Drawings  were  made  of  the  posterior  half,  great 
care  being  taken  to  orient  it  so  that  one  would  look  into  it 


No.  3-]  ACUTE    VISION  IN  VERTEBRATES. 


453 


Fig.  5.  —  White-bellied  Swallow  (Tachycineta  bicolor)  3/1. 

NH  and  NI.  Axes  of  vision  of  fovea  nasalis. 
TR  and  TL.      "  "  "      temporalis. 


along  the  axis  of  vision.  I  have  not  attempted  an  exact  repre- 
sentation of  the  area,  but  have  only  indicated  by  dotted  lines 
its  position  and  extent  as  I  have  found  it  (PI,  XXVII,  Figs.  3, 
4,  8,  9). 

The  results  of  these  injections  only  serve  to  substantiate 
Miiller's  observations  (25).  He  states  that  mammals  are  the 
only  class  of  vertebrates  which  possess,  in  the  true  sense,  a 
retinal  circulation,  while  in  many  mammals  only  a  meagre  cir- 
culation is  present  (horse  and  rabbit).  Denissenko  (32)  has 
found  an  exception  to  this  statement.  He  describes  and 
pictures  retinal  blood-vessels  in  the  eel,  which  penetrate  to  the 
outer  nuclear  layer.  This  is  the  only  exception  which  I  have 
so  far  been  able  to  find.  In  the  sections  which  I  have  of 
the  eel's  eye,  owing  to  their  thickness  I  have  not  been  able 
to  demonstrate  the  presence  of  capillaries  beyond  the  inner 
boundary  of  the  outer  nuclear  layer.  Fishes  and  amphibians 
possess  a  good  circulation  in  the  hyaloid  membrane,  while  birds 


454  SLONAKER.  [Vol.  XIII. 

and  many  reptiles  have  the  circulation  of  the  pecten.  Huschke 
126)  states  that  these  vessels  of  the  hyaloid  membrane  and  the 
pecten  correspond  to  the  retinal  vessels  in  mammals.  They  do 
not,  however,  penetrate  the  retina. 

The  photographic  representations  of  the  fovea  and  area  of 
the  different  animals  were  all  taken  with  the  same  magnification 
so  that  they  are  directly  comparable.  In  every  case  the  section 
through  the  center  of  the  fovea  was  selected.  In  some  cases 
the  section  includes  not  only  the  bottom  of  the  fovea,  but  also 
some  cells  of  the  inner  edge  of  the  area  beyond.  In  this  case 
the  bottom  of  the  fovea  is  more  or  less  obscured  by  these  cells. 
In  case  the  retina  does  not  lie  smoothly  on  the  choroid,  its 
position  is  to  be  considered  abnormal  and  due  either  to  post 
mortem  changes  or  to  the  reaction  of  the  reagents. 

Description. 

Before  giving  a  description  of  the  areas  of  acute  vision  in 
the  animals  examined,  a  few  words  may  be  necessary  regarding 
the  development,  form,  position,  and  prevalence  of  the  area  and 
fovea  centralis  in  different  vertebrates. 

In  the  development  of  a  fovea  an  area  centralis  is  first  differ- 
entiated (27).  This  stage,  according  to  Chievitz,  is  present  in 
the  human  foetus  about  the  sixth  month,  after  which  time  the 
fovea  begins  to  appear  (29).  This  takes  place  by  a  pitting  in 
of  the  vitreal  surface,  or  a  crowding  to  the  sides,  as  it  were,  of 
the  cells  in  the  center  of  the  area.  The  area  differs  from  the 
rest  of  the  retina  either  in  thickness  or  in  compactness  of  cells. 
In  some  cases  the  difference  in  thickness  is  easily  seen  and 
sharply  marked  off  (PI.  XXVII,  Figs,  i,  3,  6,  7,  8),  while 
in  others  the  increase  is  very  gradual  and  slight  (PL  XXVII, 
^'g-  5)-  The  increase  in  thickness  is  due  usually  to  a  greater 
number  of  nerve  cells,  cells  of  the  inner  and  outer  nuclear 
layers,  and  greater  length  of  the  rods  and  cones.  But  an  area 
is  not  necessarily  designated  by  an  increase  in  thickness  of 
any  of  these  layers  because  the  cells  may  be  more  numerous 
and  packed  more  closely  together.  This  is  well  illustrated  in 
the  nerve-cell  layer  of  the  frog's  area,  which  is  but  a  single  cell 


No.  3-]  ACUTE   VISION  JN  VERTEBRATES.  455 

deep  over  the  entire  retina.  In  the  center  of  the  area,  however, 
the  cells  lie  closely  together,  while  in  other  parts  of  the  retina 
they  are  some  distance  apart. 

The  rods  and  cones  in  the  area  have  a  less  diameter  and  are 
more  numerous  per  given  area  than  elsewhere.  Hence  the 
cells  which  form  their  connection  with  the  nerve  fibres  (nerve 
cells  and  cells  of  the  inner  nuclear  layer,  or  bipolar  cells)  must 
also  be  more  numerous.  But  this  is  not  the  only  reason  for  an 
increase  in  the  number  of  cells  in  the  area.  Ramon  y  Cajal 
(28)  has  described  and  pictured  the  manner  in  which  these  cells 
form  the  connection  between  the  nerve  fibres  and  the  rods  and 
cones.  Numerous  processes  (dendrites)  from  the  nerve  cells 
pass  outward  and  branch  profusely  in  the  inner  molecular  layer 
among  the  ingoing  branches  (neurites)  of  the  cells  of  the  inner 
nuclear  layer,  A  similar  relation  exists  in  the  outer  molecular 
layer  between  the  outgoing  branches  (dendrites)  of  these  cells, 
which  are  bipolar,  and  the  ingoing  branches  (neurites)  of  the 
cells  of  the  outer  nuclear  layer.  These  fine  branches  from  the 
cells  of  different  layers  only  come  in  close  relation,  and  in  no 
case  were  they  found  to  anastomose  with  each  other.  He 
divides  these  bipolar  cells  into  two  classes:  (i)  those  whose 
dendrites  come  in  contact  with  the  neurites  from  the  cone  cells; 
and  (2)  those  whose  dendrites  come  in  contact  with  neurites 
from  the  rod  cells.  In  the  periphery  of  the  retina  each  bipolar 
rod  cell  may  come  in  contact  with  from  10  to  30  rods.  Likewise 
each  bipolar  cone  cell  is  related  to  several  cones.  But  toward 
the  center  of  vision  the  number  of  rods  and  cones  which  con- 
nect with  a  single  bipolar  cell  becomes  rapidly  less,  and  in  the 
center  of  the  fovea  each  bipolar  cell  comes  in  contact  with  but 
a  single  cone.  Ramon  y  Cajal  also  finds  a  similar  relation 
existing  between  the  cells  of  the  nerve-cell  layer  and  the 
bipolar  cells  of  the  inner  nuclear  layer. 

The  number  of  cells  in  the  outer  nuclear  layer  is  directly 
dependent  on  the  number  of  rods  and  cones,  each  rod  and  cone 
having  but  a  single  nucleus.  In  fact  a  rod,  or  cone,  with  its 
nucleus,  has  long  been  considered  as  a  much  drawn-out  cell 
whose  dendritic  end  (the  rod  or  cone)  is  more  or  less  distant 
from  the  nucleus,  and  is  in  some  cases  connected  only  by  a 


456  SLONAKER.  [Vol.  XIII. 

fibre.  In  the  periphery  of  the  retina  each  rod  and  cone  nucleus 
lies  vertically  over  the  rod  or  cone  to  which  it  belongs,  and  the 
cone  nucleus  is  in  the  base  of  the  cone.  But  in  the  region  of 
the  fovea  these  nuclei  are  crowded  toward  the  periphery  and  lie 
some  distance  from  the  rods  and  cones  to  which  they  belong. 
This  causes  a  diagonal  appearance  of  these  fibres  in  a  cross 
section  of  the  retina.  The  cone  nuclei  here  lie  three  or  four 
deep.  The  rods  gradually  decrease  in  relative  number  toward 
the  area  or  fovea,  and  in  the  center  of  the  fovea  they  are  wanting, 
cones  only  being  present.  Borysiekiewicz  (30)  states,  however: 
"  Within  the  fovea  centralis  all  distinguishing  characteristics  be- 
tween the  rods  and  cones  are  lacking ;  the  so-called  cones  of  the 
fovea  entirely  resemble  the  rods  in  the  periphery  of  the  retina, 
it  is  therefore  correct  to  speak,  not  of  the  cones,  but  of  the  rods 
of  the  fovea."     Most  investigators  do  not  uphold  his  view. 

The  length  of  the  rods  and  cones  in  the  fovea  and  area 
varies  considerably  in  different  animals.  They  may  be  longer 
than  in  other  parts  of  the  retina,  as  in  the  crow  (PI.  XXIX, 
Fig.  52),  or  shorter,  as  in  the  ring-neck  plover  (PI.  XXIX,  Fig. 
47).  This  difference  in  length  of  rods  and  cones  is  shown  by 
a  greater  or  less  thickness  of  the  pigment  layer.  Dimmer  (29) 
thinks  in  the  human  macula  the  rod  and  cone  layer  is  of  about 
the  same  thickness  as  elsewhere. 

If  the  front  half  and  vitreous  humor  is  removed  from  the 
hardened  eye,  in  many  animals  the  area  is  readily  seen  as  a 
whiter  region  which  has  various  shapes  and  positions.  It  is 
never  sharply  marked  off  from  the  surrounding  retina,  but 
gradually  blends  with  it.  Its  form  may  be  circular,  oval,  or 
band-like.  In  the  latter  case  it  may  also  contain  a  circular 
area,  as  in  the  ring-neck  plover,  goose,  and  tern  (PI.  XXVII, 
Figs.  8,  12).  The  round  area  may  be  situated  on  the  nasal  side 
of  the  nerve  entrance,  in  which  case  it  is  designated  area 
nasalis ;  or  it  may  be  on  the  temporal  side,  and  designated 
area  temporalis.  Usually  at  least  one  of  these  different  kinds 
of  areas  is  present,  and  all  three  may  be  present  as  in  the  tern. 
In  many  eyes,  however,  when  examined  in  this  macroscopical 
way,  no  area  is  visible.  It  is  not  until  sections  are  made  and 
subjected  to  careful  microscopical  examination  and  measure- 


No.  3-]  ACUTE    VISION  IN  VERTEBRATES.  457 

merit  that  an  area  is  discerned.     Even  with   the  aid  of  the 
microscope  the  presence  of  an  area  is  often  very  doubtful. 

The  fovea  is  also  found  to  have  a  variety  of  forms  and  posi- 
tions. In  depth  it  may  vary  from  the  very  questionable  de- 
pression found  in  the  guinea  hen  (PI.  XXVIII,  Fig.  36)  to  the 
very  deep  pit  of  the  crow  or  hawk  (PI.  XXIX,  Figs.  48,  50,  52). 
It  has  also  been  found  to  vary  in  the  same  species,  but  this 
may  be  due  to  slight  swelling  produced  in  hardening  (pigeon, 
PI.  XXVIII,  Figs.  37,  38).  In  size  it  varies  from  the  very  broad 
fovea  in  the  human  (PI.  XXVIII,  Figs.  24,  25)  to  the  exceedingly 
sharp  depression  in  the  sparrow  hawk  (fovea  nasalis)  and  the 
horned  toad  (PI.  XXX,  Fig.  56),  Chievitz  (31,  a)  claims  that 
certain  animals  have  also  a  trough-like  fovea,  such  as  the  tern, 
etc.  I  have  not  been  able  to  procure  any  of  the  species  which 
he  describes  as  having  a  trough-like  fovea,  but  have  succeeded 
in  obtaining  a  species  from  the  same  genus.  Sterna.  In  a 
macroscopical  examination  one  would  at  once  conclude  that 
there  was  a  trough-like  fovea  present,  but  when  sections  were 
made  across  this  trough-like  appearance,  no  depression  was 
found  (PI.  XXIX,  Figs.  40,  42).  In  my  researches  I  have  seen 
nothing  excepting  the  macroscopical  appearance  which  might 
be  taken  to  indicate  the  presence  of  a  trough-like  fovea  in  any 
of  the  animals  examined.  Chievitz  has  pictured  only  the 
macroscopical  view  of  his  trough-like  fovea,  and  in  no  place 
have  I  found  that  he  has  made  cross  sections  and  microscopical 
examination.  In  the  tabulation  which  follows  I  have  used  his 
descriptions,  as  I  have  not  been  able  to  examine  the  species 
which  he  has  described. 

The  relations  of  the  fovea  correspond  to  the  positions  above 
described  for  the  area.  That  is,  we  find  a  fovea  nasalis,  as  in 
the  crow,  blue  jay,  robin,  snow-bird,  etc.,  and  z.  fovea  temporalis, 
as  in  man,  gorilla,  owl,  tern,  hawks,  etc.  One  or  two  foveas 
may  be  present,  but  in  each  case  where  two  are  present  the 
nasal  fovea  is  always  deeper  than  the  temporal. 

A  very  noticeable  and  important  difference  in  the  position  of 
the  fovea  in  various  birds  has  been  observed.  Very  little  vari- 
ation is  found  in  the  position  of  the  fovea  nasalis,  but  the  loca- 
tion of  the  fovea  temporalis  depends  wholly  upon  the  position 


458  SLONAKER.  [Vol.  XIII. 

of  the  eye  in  the  head.  As  the  eyes  are  turned  more  and  more 
forward,  the  fovea  temporalis  approaches  the  fovea  nasalis,  and 
as  binocular  vision  becomes  more  frequent,  the  nasal  fovea 
becomes  less  distinct  or  merges  into  the  temporal.  This  change 
is  shown  in  Plate  XXXI.  The  change  to  an  asymmetrical 
form,  and  the  position  of  the  lens  in  the  eyes  of  the  birds  which 
possess  the  power  of  binocular  vision,  is  also  quite  marked.  In 
many  of  these  birds,  as  the  tern  and  white-bellied  swallow 
(PI.  XXIX,  Figs.  40,  41,  46,  and  PI.  XXXI,  Figs.  64,  65), 
where  the  fovea  nasalis  is  sharp  and  deep  and  the  fovea  tem- 
poralis quite  shallow,  the  eyes  are  almost  symmetrical.  But 
in  those  birds  which  use  binocular  vision  more  as  shown  by  a 
greater  depth  of  the  temporal  fovea,  as  in  the  hawks  (PI. 
XXIX,  Figs.  48-51,  and  PI.  XXXI,  Figs,  m,  67),  the  eye 
becomes  more  asymmetrical,  and  finally  reaches  the  most 
irregular  form  in  the  owl  (PI.  XXXI,  Fig.  68),  where  binocular 
vision  only  occurs. 

Various  combinations  of  area  and  fovea  are  found  in  different 
animals.  The  most  simple  is  a  single  fovea  surrounded  by  a 
circular  area,  as  in  the  primates  and  most  birds.  Again  we 
find  a  simple  fovea  surrounded  by  a  round  area  which  is  con- 
tinuous with  a  band-like  area  extending  horizontally  across  the 
retina,  as  in  the  goose  and  ring-neck  plover.  One  or  two  foveae 
may  be  present,  each  surrounded  by  a  round  area  and  connected 
by  a  short,  slight,  band-like  area,  as  in  the  sparrow  hawk,  red- 
tailed  buzzard,  and  kingfisher.  The  most  complex  combination 
which  I  have  found  is  in  the  tern.  Here  the  fovea  temporalis 
surrounded  by  a  small  round  area  is  not  connected  with  the 
band-like  area  which  extends  horizontally  across  the  retina,  and 
near  its  middle  widens  out  into  a  round  area  surrounding  the 
fovea  nasalis  (PI.  XXVII,  Fig.  12). 

In  my  researches  I  have  been  able  to  examine  93  different 
species,  of  which  18  were  mammals,  41  birds,  6  reptiles,  3  am- 
phibians, and  25  fishes.  In  some  cases  the  results  were  doubt- 
ful, and  sufficient  material  was  not  available  to  ascertain  all 
points  with  certainty.  Such  cases  I  have  indicated.  Many  of 
the  species  examined  have  been  observed  by  others,  in  which 
case  I  have  always  aimed  to  give  credit  to  the  first  observer. 


No.  3-]  ACUTE    VISION  IN  VERTEBRATES.  459 

In  the  animals  which  I  have  examined  the  space  for  the 
observer  is  left  blank,  excepting  when  the  species  has  been 
previously  examined,  in  which  case  I  have  inserted  (S)  after 
the  name  of  the  first  investigator. 

In  the  following  tabulation  I  have  adopted  the  form  of 
Chievitz,  and  have  endeavored  to  present  the  results  of  all 
investigators  up  to  the  present  time.  Some  results  have 
necessarily  been  omitted,  as  the  investigators  in  their  descrip- 
tions have  given  only  the  common  name  and  not  the  species  of 
the  animal. 


460 


SLONAKER. 


[Vol.  XIII. 


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No.  3-]  ACUTE   VISION  IN  VERTEBRATES.  47 1 


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[Vol.  XIII. 


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Found 

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Pomatomus  saltatrix 
Stromateus  triacanthus 
Perca  flavescens 
Centropristis  striatus 
Stenotomus  chrysops 
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Pediculati 

Lorphius  piscatorius 

No.  3.] 


ACUTE    VISION  IN  VERTEBRATES. 


473 


The  following  tabulation,  condensed  from  the  foregoing,  will 
show  at  a  glance  the  prevalence  of  an  area  and  fovea  centralis 
in  the  different  classes  of  vertebrates. 


It 

II 

2: 

< 
<   a 

1^ 

•< 

0  fa 
55 

Area 

Fovea 

One 

Round 

Two 
Round 

Band- 
like 

One 

Simple 

Two 
Simple 

51 

Mammals 

10 

38 

31 

8 

18 

102 

Birds 

0 

I 

59 

II 

36 

7^ 

II 

22 

25 

Reptiles 

3(?) 

17 

20 

3 

6 

2 

14 

Amphibians 

3 

" 

3 

8 

2 

30 

Fishes 

10 

25 

20 

5 

Mammals. 

Mammals  as  a  class  are  characterized  by  the  absence  of  a 
fovea,  the  primates  being  the  only  ones  in  which  it  has  been 
found.  As  a  rule  an  area  is  present,  though  in  some  cases 
even  an  area  has  not  been  demonstrated.  H.  Miiller  (13)  says: 
"  In  mammals  there  is  at  least  an  area  centralis  present  which 
approaches  in  structure  the  yellow  spot,  and  is  made  known  by 
a  similar  course  of  the  blood-vessels  as  in  man."  If  such  is 
the  case,  I  have  failed  in  some  instances  to  demonstrate  the 
presence  of  such  an  area. 

In  some  mammals  the  area  is  readily  seen  with  the  naked 
eye,  but  in  the  majority  of  those  I  have  been  able  to  examine 
such  is  not  the  case.  In  many  instances  vertical  and  horizontal 
sections  have  to  be  made  and  subjected  to  microscopical  exam- 
ination and  measurement  before  a  thickening  or  an  arrangement 
of  cells  indicating  an  area  is  found.  In  some  the  very  slight 
thickening  is  marked  also  by  an  increase  in  thickness  of  the 
tapetum. 

I  will  now  proceed  to  a  more  detailed  description  of  the 
area  and  fovea  in  the  mammals  which  I  have  studied.  I  shall 
not,  however,  enter  into  the  histological  arrangement  of  the 
cells. 


474  SLONAKER.  [Vol.  XIII. 


_4<  ^•hcrr^a^j 


Human  and  Gorilla. 

The  fovea  and  macula  lutea  are  readily  seen,  located  about 
4  mm.  toward  the  naoft^  side  of  the  entrance  of  the  optic  nerve. 
The  macula  is  rather  sharply  marked  off  from  the  surrounding 
retina,  and  is  of  small  extent  compared  with  other  mammals. 
The  blood-vessels  in  either  of  these  cases  were  not  injected, 
but  they  could  be  traced  as  far  as  represented  in  PI.  XXVII, 
Figs.  I,  2.  Figs.  24,  25,  PI.  XXVIII,  represent  horizontal  sec- 
tions through  a  child's  eye  and  an  adult's  respectively.  The 
foveola  described  by  Dimmer  (29)  is  much  more  noticeable 
in  the  adult  (Fig.  25)  than  in  the  child's  fovea.  In  the  case  of 
the  gorilla,  which  was  about  nine  hours  post  mortem,  folds  had 
formed  about  the  fovea  so  that  its  appearance  is  not  well  rep- 
resented in  sections.  PI.  XXVIII,  Fig.  26,  represents  the 
horizontal  section  and  Fig.  27  the  vertical  section  through 
the  center  of  the  fovea. 

Rabbit  (Lepus  sylvaticus). 

The  nerve  entrance  is  readily  seen  above  the  center  and  a 
little  toward  the  temporal  side.  From  it  two  large  bundles  of 
nerve  fibres  branch  out  horizontally.  In  the  injected  specimen 
the  blood-vessels  are  seen  to  lie  in  these  bundles,  and  do  not 
branch  over  the  rest  of  the  retina.  The  band-like  area  is  seen 
to  extend  horizontally  across  the  retina,  immediately  below  the 
nerve  entrance,  and  to  gradually  fade  out  just  before  reaching 
the  ora  serrata.  It  is  from  ^  to  i  mm.  broad  (PI.  XXVII, 
Fig.  13). 

Rat  (Mus  rattus). 

I  have  not  succeeded  in  demonstrating  the  presence  of  a 
definite  area  in  this  animal. 

WoODCHUCK  (Arctomys  monax). 
Red  Squirrel  (Sciurus  hudsonicus). 
Fox  Squirrel  (Sciurus  niger). 
Chipmunk  (Tamias  striatus). 

No  area  is  visible  to  the  naked  eye,  but  in  horizontal  and 
vertical  sections  a  slightly  thicker  oblong  or  oval  area  is  dis- 


No.  3-]  ACUTE    VISION  IN  VERTEBRATES.  475 

cernible.  This  I  have  called  the  area  centralis.  It  is  situated 
near  the  center  of  the  retina,  but  slightly  above  and  toward 
the  temporal  side.  The  nerve  entrance  is  very  noticeable  and 
of  unusual  shape.  The  nerve  is  flattened  out  fan-like  just 
before  piercing  the  sclerotic,  so  that  the  papilla  is  narrow  and 
elongated.  PI.  XXVII,  Fig.  14,  represents  the  entrance  of 
nerve  and  the  position  of  the  area  as  nearly  as  I  can  ascertain 
it  in  Sciurus  niger. 

Bat  (Vespertilio  subulatus). 

I  have  not  been  able  with  the  material  at  hand  to  demonstrate 
an  area. 

Sheep  (Ovis  avies). 

Chievitz  (31,  b)  has  described  this  area  as  not  visible  to  the 
naked  eye,  round,  about  4  mm.  in  diameter,  and  located  about 
8  mm.  toward  the  temporal  side  of  the  nerve  entrance.  I  have 
examined  more  than  20  eyes,  and  in  every  case  find  a  white, 
band-like  region,  about  1-2  mm.  broad,  extending  horizontally 
across  the  retina,  gradually  becoming  invisible  to  the  naked 
eye  just  before  reaching  the  ora  serrata.  It  compares  favorably 
in  every  respect  with  the  area  centralis  of  the  cow.  It  lies 
above  the  nerve  entrance,  which  is  below  the  center  and  toward 
the  temporal  side.  PI.  XXVII,  Fig.  8,  represents  the  position 
and  extent  of  the  area  and  its  relation  to  the  blood-vessels  and 
nerve  entrance  in  the  left  eye. 

Cow  (Bos  taurus  domesticus). 

A  horizontal  band-like  area  1-2  mm.  broad  is  present,  hav- 
ing the  same  general  relation  to  the  nerve  entrance  and  blood- 
vessels as  found  in  the  sheep  (PI.  XXVII,  Fig.  6). 

Pig  (Sus  domesticus). 

A  band-like  area  about  i  mm.  broad  passes  horizontally  across 
the  retina,  and  has  the  same  relation  to  the  blood-vessels  and 
nerve  as  that  described  for  the  sheep  and  cow.  The  nerve 
entrance  is  nearer  the  center  of  the  retina  (PI.  XXVII, 
Fig-  9)- 


476  SLONAKER.  [Vol.  XIII. 

Horse  (Equus  caballus). 

The  band-like  area  is  here  very  broad,  5-7  mm.,  and  extends 
horizontally  across  the  retina.  The  nerve  entrance  is  below 
the  center  and  slightly  toward  the  temporal  side.  The  blood- 
vessels are  very  meagre,  and,  according  to  Miiller,  extend  over 
but  a  small  portion  of  the  retina,  leaving  the  area  centralis  and 
the  entire  upper  part  of  the  retina  free  from  blood-vessels.  The 
blood-vessels  are  usually  not  visible  unless  they  are  injected 
(PI.  XXVII,  Fig.  7). 

Cat  (Felis  catus  domesticus). 

Chievitz  (31,  c)  has  described  the  area  as  round  and  not  visi- 
ble to  the  naked  eye  and  located  toward  the  temporal  side  of 
the  nerve  entrance.  Ganser  (19)  has  described  it  as  round, 
and  Krause  (18)  has  stated  that  the  cat  possesses  a  fovea  as 
well  as  an  area.  The  retinal  blood-vessels  (PI.  XXVII,  Fig.  4) 
would  indicate  as  much  as  those  of  the  sheep  or  cow  the  pres- 
ence of  a  band-like  area.  Or  the  finer  branches  radiating 
toward  a  common  point  on  the  temporal  side  of  the  nerve 
entrance  would  suggest  a  round  area  similarly  located  as  that 
described  by  Chievitz.  In  most  cases  a  region  similar  to  that 
indicated  by  the  dotted  lines  and  having  the  same  macroscopical 
appearance  as  an  area  is  observed.  This  appearance  may  be 
due  to  the  tapetum  lying  behind.  The  lower  margin  of  this  area- 
like region  corresponds  very  closely  with  that  of  the  tapetum. 
In  sections  I  have  not  found  a  well-defined  round  area,  but  a 
general  thickening  over  the  greater  part  of  the  region  indicated. 

Skunk  (Mephitis  mephitica). 
Mink  (Putorius  vison). 

No  area  is  visible  to  the  naked  eye,  but  in  horizontal  and 
vertical  sections  an  oblong  or  oval  thickening  is  found  located 
a  little  above  and  on  the  temporal  side  of  the  nerve  entrance, 
which  in  these  animals  is  central. 

Fox  (Vulpes  vulpes). 

A  horizontal,  band-like  region  extending  across  the  retina 
just  above  the  nerve  entrance  may  be  seen  with  the  naked  eye 


No.  3-]  ACUTE    VISION  IN  VERTEBRATES.  477 

(PL  XXVII,  Fig.  5).  In  the  representation  the  retinal  vessels 
were  not  injected,  and  the  smaller  branches  could  not  be  accu- 
rately made  out.  In  cross  sections  only  a  slight  thickening  of 
the  retina  is  noticed,  and  the  lower  edge  of  the  indicated  region 
corresponds  with  the  lower  margin  of  the  tapetum. 

Dog  (Canis  familiaris). 

The  retinal  blood-vessels  (PL  XXVII,  Fig.  3)  indicate  the 
presence  of  a  band-like  area.  Again  the  finer  and  more  numer- 
ous branches  radiating  toward  a  common  spot  on  the  temporal 
side  of  the  nerve  entrance  points  to  the  presence  of  a  round 
area.  Neither  are  visible  to  the  naked  eye,  but  Chievitz  (31,  b) 
has  described  the  presence  of  a  round  area  in  this  latter  position. 
I  have  not  succeeded  in  demonstrating  it. 

Birds. 

Birds  are  characterized  by  the  presence  of  a  fovea,  although 
a  few  cases  are  very  doubtful  (hen  and  guinea  hen).  Chievitz 
says  (31,  c)  that  at  least  a  round  area  is  always  present  which 
regularly  possesses  a  fovea,  sometimes  very  clearly  seen,  and 
in  other  cases  so  shallow  as  to  be  very  doubtful  (duck  and 
hen). 

Where  but  a  single  fovea  is  present  the  position  and  form 
are  so  similar,  as  shown  in  the  tabulation,  that  a  large  number 
may  be  described  together.  As  a  rule  it  is  situated  about  the 
center  of  the  retina,  a  short  distance  above  and  toward  the  nasal 
side  of  the  optic  nerve  entrance.  The  nerve  entrance  is  always 
more  or  less  obscured  from  view  by  the  pecten,  which  extends 
obliquely  from  the  point  of  entrance  downward  and  forward,  so 
that  a  line  joining  the  fovea  and  nerve  entrance  forms  about  a 
right  angle  with  the  pecten  (PL  XXVII,  Figs.  17,  23).  The 
fovea,  with  but  few  exceptions  which  will  be  described  separately, 
is  surrounded  by  a  simple  round  area  more  or  less  sharply 
marked  off  from  the  surrounding  retina.  The  fovea  varies 
considerably  in  depth.  In  the  tabulation  I  have  classified  them 
as  deep,  medium,  and  shallow. 

Most  birds  possess  a  deep  and  well-defined  fovea,  as  seen  in 
the  following: 


478  SLO MAKER.  [Vol.  XIII. 

Robin  (Merula  migratoria,  PI.  XXVII,  Fig.  17,  and  PI.  XXVIII,  Fig.  28). 

Blue-Bird  (Sialia  sialis,  PI.  XXVIII,  Fig.  29). 

Kinglet  (Regulus  satrapa,  PI.  XXVIII,  Fig.  30). 

Sx\ow-BiRD  (Junco  hyemalis,  PL  XXVIII,  Fig.  31). 

Crow  (Corvus  americanus,  PI.  XXIX,  Fig.  52). 

Blue  Jay  (Cyanocitta  cristata,  PL  XXIX,  Fig.  53). 

Night  Heron  (Nycticorax  nycticorax,  PL  XXVII,  Fig.  23). 

A  number  of  birds  possess  a  medium  fovea,  as  seen  in  the 
pigeon  (Columba  livia  domestica,  PI.  XXVIII,  Figs.  37,  38). 
It  is  readily  observed  surrounded  by  a  well-defined  area.  It 
varies  somewhat  in  depth  in  the  same  species,  as  is  shown  in 
this  case.  Fig.  37  represents  a  medium  fovea,  while  Fig.  38 
would  be  classed  as  shallow. 

Most  of  the  Gallinas  which  I  have  examined  possess  medium 
to  very  shallow  fovea.  The  quail  (Colinus  virginianus)  and  the 
partridge  (Bonasa  umbellus)  each  possess  a  medium  fovea, 
while  in  the  turkey  (Meleagris  gallopavo,  PI.  XXVIII,  Fig.  35) 
and  the  guinea  hen  (Numida  pucherani,  PI.  XXVIII,  Fig.  36) 
it  is  shallow.  In  the  last  case  the  depression  is  so  slight  as  to 
scarcely  deserve  the  name  of  fovea.  Chievitz  mentions  an  area 
nasalis  and  a  questionable  fovea  in  the  hen  (Gallus  doraesticus). 
I  have  succeeded  in  finding  only  a  very  slight  thickening. 

Screech  Owl  (Megascops  asio). 
Barred  Owl  (Syrnium  nebulosum). 

These  owls  possess  a  single  deep  fovea  surrounded  by  a 
sharply  defined  round  area  which  differs  from  those  just 
described  only  in  position.  It  is  located  on  the  temporal  side 
and  above  the  nerve  entrance  in  such  a  position  as  to  function 
in  binocular  vision.  The  nerve  entrance  is  similar  in  position 
to  that  of  other  birds,  but  the  pecten  is  much  smaller  in  pro- 
portion to  the  size  of  the  eye  (PI.  XXIX,  Fig.  55,  and  PI. 
XXVII,  Fig.  10). 

Goose  (Anser  cinereus  domesticus). 

The  goose  possesses  a  shallow  fovea  nasalis  surrounded  by  a 
round  area  situated  on  a  band-like  area  extending  horizontally 
through  the  retina.  The  fovea  and  round  area  are  easily 
observed  with  the  naked  eye,  but  the  band-like  area  is  much 


No.  3-]  ACUTE    VISION  IX  VERTEBRATES.  ^jg 

less  distinct.  Vertical  sections  across  this  area  show  only  a 
slight  increase  in  thickness,  both  on  the  nasal  and  temporal  side 
of  the  fovea  (PL  XXVIII,  Figs.  32-34)- 

Tame  Duck  (Anas  boschas  domesticus). 
Surf  Duck  (Oidemia  deglandi). 

Similar  relations  exist  here  as  in  the  goose.  The  fovea  is 
quite  shallow,  and  is  surrounded  by  a  distinct  round  area  which 
is  situated  on  a  band-like  horizontal  area  (PI.  XXVIII,  Fig.  39). 

Rixg-Xeck  Plover  (iEgialitis  semipalmata). 

A  very  distinct  band-like  area  is  seen  passing  obliquely 
through  the  retina.  A  dark  line,  resembling  a  trough-like 
fovea,  extends  almost  the  full  length  through  the  center  of  this 
area.  Cross  sections  reveal,  however,  no  trough-like  fovea. 
The  single  fovea  nasalis,  surrounded  by  a  sharply  bounded 
round  area,  is  observed  located  about  the  middle  of  the  band- 
like area.  It  is  of  medium  depth  and- readily  seen  by  the  naked 
eye  (PI.  XXVII,  Fig.  20,  and  PI.  XXIX,  Fig.  47). 

Sparrow  Hawk  (Falco  sparverius). 

A  fovea  nasalis  and  a  fovea  temporalis,  each  surrounded  by 
a  sharp  round  area  connected  by  a  short  band-like  area,  are 
easily  observed.  The  fovea  nasalis  is  very  deep  and  sharp  and 
is  situated  about  the  center  of  the  retina.  The  fovea  tempo- 
ralis, somewhat  shallower,  is  situated  near  the  ora  serrata  about 
the  same  distance  from  the  nerve  entrance  as  the  fovea  nasalis, 
but  in  a  lower  plane.  The  area  temporalis  is  likewise  smaller 
than  the  area  nasalis.  The  band-like  area  is  not  sharply 
bounded,  is  of  slight  thickness,  and  extends  only  between  the 
two  round  areas.  The  fovea  temporalis  is  similar  in  position 
to  that  of  the  owl,  and  the  fovea  nasalis  to  that  of  the  crow, 
robin,  etc.  (PI.  XXVII,  Fig.  19,  and  PI.  XXIX,  Figs.  48,  49). 

Red-Tailed  Buzzard  (Buteo  borealis). 

Almost  the  same  conditions  exist  as  found  in  the  sparrow 
hawk,  excepting  the  two  foveae  are  relatively  nearer  together 
(PL  XXVII,  Fig.  II,  and  PL  XXIX,  Figs.  50,  51). 


480  SLONAKER.  [Vol.  XIII. 

Kingfisher  (Ceryle  alcyon). 
The  same   conditions   exist  as  are  found  in  the  hawk  (PI. 
XXIX,  Figs.  44,  45). 

White-Bellied  Swallow  (Tachycineta  bicolor). 

A  fovea  nasalis  and  a  fovea  temporalis  are  easily  seen,  each 
surrounded  by  a  round  area  situated  on  a  band-like  area  extend- 
ing obliquely  across  the  retina.  The  positions  of  the  foveae 
are  very  similar  to  those  described  in  the  hawks.  The  fovea 
and  area  temporalis  are  likewise  smaller,  and  are  situated  nearer 
the  ora  serrata  than  the  fovea  and  area  nasalis  in  the  hawks. 
The  area  and  fovea  nasalis  are  shown  in  PI.  XXIX,  Fig.  46. 

Common  Tern  (Sterna  hirundo). 

In  this  case  both  nasal  and  temporal  foveas  surrounded  by 
round  areas  are  present,  and  in  addition  a  band-like  area.  The 
area  nasalis  is  located  on  the  band-like  area  about  the  center  of 
the  retina,  but  the  area  temporalis  is  above  the  band-like  area, 
and  apparently  in  no  way  connected  with  it.  A  dark  line, 
resembling  a  trough-like  depression,  extends  through  the  center 
of  the  band-like  area,  through  the  fovea  nasalis,  and  terminates 
near  the  entrance  of  the  optic  nerve  (PI.  XXVTI,  Fig.  12).  A 
cross  section  of  this  area,  given  in  PI.  XXIX,  Figs.  40,  42, 
fails  to  demonstrate  such  a  depression.  The  temporal  end  of 
the  band-like  area  widens  and  soon  becomes  indistinct.  The 
fovea  temporalis  is  very  shallow  and  might  be  overlooked.  It 
is  located  near  the  ora  serrata  a  little  above  the  median  hori- 
zontal plane.  The  fovea  nasalis  is  deep  and  easily  observed 
(PL  XXIX,  Figs.  41,  43). 

Reptiles. 

In  the  tabulation  twenty-five  species  are  mentioned.  All  but 
three  are  described  as  having  an  area,  and  these  three  are  ques- 
tionable. Eight  of  the  number  possess  a  well-defined  fovea, 
while  two  are  doubtful. 

In  snakes  an  area  seems  to  be  the  rule.  In  the  three 
species  I  have  examined,  the  retina  was  not  sufficiently  well 


No.  3]  ACUTE    VISION  IN  VERTEBRATES.  48 1 

preserved  to  make  certain  the  presence  of  an  area.  It  is  only 
visible  in  sections. 

In  the  lizard  an  area  has  been  described  in  every  case,  and  a 
fovea  in  all  but  two,  which  are  doubtful.  The  only  lizard  which 
I  have  examined,  the  horned  toad  (Phrynosoma  cornutum),  pos- 
sesses a  deep  and  sharp  fovea,  situated  on  a  broad  band-like 
area.  The  fovea  is  situated  about  the  center  of  the  retina,  just 
above  the  entrance  of  the  optic  nerve,  which  is  marked  by  a 
slender  conical  pecten.  The  band-like  area  is  broadest  in  the 
region  of  the  fovea,  and  extends  horizontally  across  the  retina. 
A  dark  line  extends  about  i  mm.  to  either  side  from  the  fovea 
and  gives  the  appearance  of  a  trough-like  fovea,  as  seen  in  the 
tern,  but  cross  sections  reveal  no  depression.  The  band-like 
area  gradually  becomes  indistinct  some  distance  from  the  ora 
serrata  (PI.  XXVII,  Fig.  15,  and  PI.  XXX,  Figs.  56,  57). 

In  the  turtles  only  an  area  has  been  found  which  is  oval  or 
round  in  shape,  and  lies  about  the  center  of  the  retina,  just 
above  the  nerve  entrance.  It  is  not  visible  to  the  naked  eye, 
and  in  sections  is  noticed  rather  as  a  closer  arrangement  of 
the  cells  than  as  a  thickening.  PI.  XXX,  Fig.  61,  represents 
a  section  through  the  area  of  Chelydra  serpentina.  A  repre- 
sentation of  the  entire  retinal  section  would  be  necessary  to 
show  any  difference  in  thickness.  In  an  injected  specimen  of 
Chelopus  insculptus,  a  short  and  seemingly  rudimentary  blood- 
vessel was  noticed  (PI.  XXVII,  Fig.  16)  which  seemed  to  be 
an  approach  to  a  retinal  circulation.  In  the  other  eye  it  was 
not  so  long  but  similarly  located. 

In  the  crocodiles  Chievitz  has  described  and  pictured  a  band- 
like area  and  shallow  trough-like  fovea  which  extend  horizontally 
through  the  entire  retina.  I  have  not  been  able  to  examine  any 
species  of  this  order. 

Amphibians. 

The  presence  of  an  area  and  absence  of  a  fovea  seems  to  be 
the  rule.  Hulke  and  Chievitz,  however,  have  described  a  shal- 
low fovea  in  Bufo  vulgaris  and  Bufo  calamitia,  though  in  some 
cases  it  is  wanting.  I  have  found  a  band-like  area  in  Bufo 
lentiginosus,  Rana  virescens  and  catesbiana.     It  is  not  visible 


482  .     SLONAKER.  [Vol.  XIII. 

to  the  naked  eye  and  is  demonstrated  only  in  vertical  sections 
by  a  slight  and  gradual  increase  in  thickness,  principally  in  the 
inner  nuclear  layer  and  in  the  closer  arrangement  of  the  nerve 
cells.  The  position  of  the  area  is  outlined  in  PI.  XXVII, 
Fig.  18,  as  found  in  Rana  catesbiana.  PI.  XXX,  Fig.  62, 
represents  the  vertical  section  through  the  area. 

Fishes. 

Fishes  seem  to  be  characterized,  as  a  rule,  by  the  absence  of 
both  a  fovea  and  a  well-defined  area.  Nothing  is  visible  to  the 
naked  eye  excepting  in  a  few  cases,  which  will  receive  special 
mention.  If  sections  of  the  eye,  however,  are  subject  to  micro- 
scopical measurement,  an  oblong  or  oval  region,  slightly  thicker 
than  the  rest  of  the  retina,  is  found  located  on  the  temporal 
side  and  a  little  above  the  center.  In  fact,  the  whole  upper 
half  of  the  retina  is  somewhat  thicker  than  the  lower  half  in 
all  fishes  which  I  have  examined.  That  region  indicated  above, 
however,  is  the  thickest,  and  I  have  designated  it  the  area 
centralis.  It  also  corresponds  in  position  to  that  of  the  fovea 
when  a  fovea  is  present.  Some  of  the  material  at  hand  was  not 
sufficient  to  demonstrate  clearly  the  presence  of  such  an  area. 
Such  cases  I  have  indicated  as  doubtful.  PI.  XXX,  Fig.  63, 
represents  a  section  through  the  area  of  the  flounder  (Para- 
lichthys  dentatus),  but  no  increase  in  thickness  is  visible  in  so 
small  a  portion  of  the  retina. 

Krause  (37)  has  described  the  presence  of  a  round  area  and 
shallow  fovea  in  Syngnathus  typhle,  and  Carriere  (16)  has 
described  and  pictured  a  similar  area  and  fovea  in  Hippocampus. 
Gulliver  (15)  has  described  a  round  area  and  shallow  fovea  in 
Pagellus  centrodonpus  (.?).  Schiefferdecker  (38)  has  described 
a  similar  area  and  fovea  in  Pleuromectes  platessa.  I  have  not 
been  able  to  procure  any  of  these  species,  but  have  found  an 
area  and  fovea  in  another  species. 

Pipefish  (Siphostoma  fuscum). 

The  eye  of  this  fish  being  so  small,  I  have  not  attempted  a 
macroscopical  examination.     The  area  and  fovea  are,  however, 


No.  3-]  ACUTE    VISION  IN  VERTEBRATES.  483 

probably  visible  to  the  naked  eye.  In  horizontal  sections  the 
area  and  fovea  may  be  readily  seen.  The  fovea  is  broad  and 
shallow  when  compared  with  that  of  most  birds  and  some 
reptiles.  It  is  located  on  the  temporal  side,  about  midway 
between  the  nerve  entrance  and  the  ora  serrata,  and  a  little 
above.  A  horizontal  section  through  the  nerve  passes  through 
the  area  below  the  fovea,  as  shown  in  PI.  XXX,  Fig.  59  (i). 
A  section  through  the  fovea  is  shown  in  Fig.  58  (i). 

Physiological. 

In  order  to  make  a  physiological  comparison  of  the  areas  of 
acute  vision  in  the  different  vertebrates,  the  exact  function  of 
the  different  elements  of  the  retina  must  be  known.  Most 
physiologists  agree  on  the  functions  of  all  the  elements  except- 
ing the  rods  and  cones.  All  are  agreed,  however,  that  the  rods 
and  cones  are  the  elements  which  give  the  sensation  of  sight, 
but  just  the  function  of  each  is  very  obscure.  The  source  of 
information  regarding  the  functions  of  the  rods  and  cones  has 
necessarily  been  confined  to  man.  When  this  has  been  finally 
settled,  a  more  accurate  comparison  of  the  powers  of  sight  in 
the  different  vertebrates  can  be  made. 

A  great  many  theories  have  been  advanced  regarding  the 
functions  of  the  rods  and  cones,  and  as  these  theories  cannot 
be  fully  verified  or  tested  by  physiological  experiments,  they 
will  have  to  be  accepted  as  such. 

What  the  changes  are  which  take  place  in  the  retina  during 
an  act  of  sight  had  long  been  a  mystery  till  the  visual  purple 
was  discovered  in  the  rod  and  cone  layer  by  Boll.  This,  how- 
ever, sufficed  for  only  a  short  time,  as  it  was  soon  found  that 
the  cones  possessed  no  visual  purple,  or  at  least  none  could  be 
demonstrated  in  them.  Since  the  cones  are  the  only  sensitive 
elements  in  the  fovea,  some  other  photo-chemical  substance 
must  be  present  in  them.  The  theories  of  Young-Helmholtz, 
Herring,  Mrs.  Franklin,  etc.,  agree  generally  in  the  functions 
of  the  rods  and  cones,  but  differ  in  the  photo-chemical  sub- 
stance and  its  change  in  an  act  of  sight.  Since  the  theories  of 
Young-Helmholtz  and  Herring  do  not  ascribe  different  func- 


484  SLONAKER.  [Vol.  XIII. 

tions  to  the  rods  and  cones,  I  shall  not  refer  to  them  further. 
Mrs.  Franklin  (39)  bases  her  theory  on  carefully  conducted 
experiments  testing  the  sensitiveness  of  different  regions  of 
the  retina  to  various  colors  and  intensities  of  light.  She 
assumes  the  presence  of  two  kinds  of  molecules  in  the  photo- 
chemical substance  of  the  retina:  (i)  gray  molecules  which 
give  rise  to  the  sensation  of  gray;  (2)  color  molecules  which 
have  been  differentiated  from  the  gray,  and  whose  atoms  of 
the  external  layer  are  arranged  in  three  directions  at  right 
angles  to  each  other.  These  give  the  sensation  of  color.  She 
would  thus  attribute  to  the  rods  the  perception  of  uncolored 
light,  for  she  says  (40,  a) :  "  In  the  very  eccentric  part  of  the 
retina  the  differentiation  of  the  color  molecule  out  of  the  gray 
molecule  has  not  taken  place;  these  parts  of  the  retina  are 
chiefly  useful  to  us  in  warning  us  of  danger  from  moving 
insects  and  other  enemies,  and  for  this  the  power  to  detect  dif- 
ferences of  brightness  is  sufficient."  Again  (41):  ^^  Only  the 
cones  are  sensitive  to  variations  of  color;  they  must  be  extremely 
sensitive  to  variations  of  intensity  in  white  light  as  well,  — 
otherwise  the  fovea  would  not  be  the  place  with  which  we  make 
out  the  minutest  variations  of  line  and  shade  in  an  intricate 
drawing.  If  the  cones  only  give  color,  they  do  not  give  color 
only."  Her  experiments,  as  well  as  those  of  Konig  (44), 
further  show  that  the  fovea  is  blind  to  blue,  and  is  not  able  to 
perceive  other  colors  when  the  illumination  is  faint,  seeing  them 
only  as  "different  intensities  of  gray"  (40,  b).  In  color-blind 
people  she  finds  that  they  are  blind  in  the  center  of  the  fovea, 
but  have  come  to  use  a  small  spot  on  the  edge  of  the  fovea  as 
the  point  of  acute  vision  (42).  The  maximum  sensitiveness 
of  the  retina  to  faint  impression  is  found  to  be  about  25°  from 
the  fovea  where  it  is  four  times  as  sensitive,  and  at  50°  it  is 
still  twice  as  sensitive  as  the  fovea.  These  gray  and  color 
molecules  are,  of  course,  only  theoretical  and  cannot  be  demon- 
strated. The  gray  molecules,  without  doubt,  correspond  to 
the  visual  purple  of  other  writers,  which  is  found  only  in  the 
rods.  The  results  of  the  various  experiments  on  the  sensitive- 
ness of  the  retina  to  different  colors  correspond  closely  with 
the  arrangement  of  the  rods  and  cones.     In  the  center  of  the 


No.  3-]  ACUTE    VISION  IN  VERTEBRATES.  485 

fovea,  where  only  cones  exist,  colors  are  most  easily  perceived, 
while  in  the  periphery,  where  there  are  few  cones,  it  is  difficult 
to  distinguish  them. 

M.  H.  Parinaud  (43)  has  found  by  experiment  on  the  excised 
retina  that  the  visual  purple  (hence  the  rods)  cannot  be  demon- 
strated nearer  the  center  of  the  fovea  than  two  millimeters. 
From  this  place  the  rods  are  found  to  increase  in  number 
toward  the  periphery  and  the  cones  to  decrease. 

Again,  the  retina  being  four  times  as  sensitive  to  faint 
impressions  25°  from  the  fovea  as  at  the  fovea,  and  since  at 
this  distance  the  rods  are  far  more  numerous  than  the  cones, 
we  can  consider  the  functions  of  the  rods  fairly  well  determined 
to  be  the  perception  of  diffuse  and  gray  lights. 

To  sum  up:  (i)  the  rods  and  cones  are  the  sensitive  elements 
of  sight;  (2)  the  rods  give  us  the  sensation  of  gray,  while  the 
cones  give  us  the  sensation  of  color  and  gray;  (3)  the  rods  are 
more  sensitive  to  faint  impressions  than  the  cones;  (4)  the 
elements  of  the  other  layers  form  the  connection  with  the  optic 
nerve. 

With  this  in  view  concerning  the  functions  of  the  retinal 
elements  in  man,  and  supposing  the  functions  to  be  the  same 
in  the  other  vertebrates,  a  physiological  comparison  may  be 
attempted. 

Quite  a  difference  is  noticed  in  the  relative  thickness  of  the 
layers  of  the  retina  of  the  different  vertebrates.  This  is  shown 
diagrammatically  in  PI.  XXVII,  Fig.  22.  The  layers  which 
exhibit  the  greatest  difference  are  the  inner  and  outer  nuclear 
layers  and  the  rod  and  cone  layer.  In  mammals  the  outer 
nuclear  layer  is  much  thicker  than  the  inner,  while  in  birds, 
reptiles,  amphibians,  and  fishes  the  reverse  is  true. 

The  layer  which  shows  the  greatest  diversity,  however,  is 
that  of  the  rods  and  cones.  A  great  difference  exists  in  their 
size,  length,  shape,  and  relative  number.  Fishes  possess  the 
longest  rods,  while  amphibians  have  not  only  long  rods,  but 
also  the  thickest  found  in  the  vertebrates.  The  rods  of  mam- 
mals are  long  but  very  slender,  while  in  birds  they  are  compar- 
atively short  and  thick.  The  cones  are  the  longest  and  most 
slender  in  some  of  the  reptiles  (chameleon),  and  of  greatest 


486 


SLONAKER. 


[Vol.  XIII. 


diameter  in  the  mammals.  They  are  about  the  same  length  in 
mammals,  birds,  and  amphibians,  while  in  fishes  they  are 
shorter.  In  birds  the  diameter  of  the  cones  approaches  very 
closely  to  that  found  in  the  reptiles.  The  following  tabulation 
of  measurements  compiled  from  Miiller's  descriptions  (20)  of 
the  rods  and-  cones  of  different  animals  will  make  clear  these 
relations.     These  measurements  are  in  millimeters. 


Rods 

Cones 

Diameter 

Length 

Diameter 

Length 

Human 

.OOI5-.OO18 

.04-.06 

.OO4-.O06 

.032-.036 

Pigeon 

.O026-.OO33 

.02-.028         .001-005 

.O25-.03 

Chameleon 

.00 1 -.00 1 3 

.06-. 08 

Frog 

.006-007 

.04-.06 

.005 

.02-028 

Perch 

.0026 

.04-.05 

.008-012 

The  diameter  of  the  rods  and  cones  is  of  great  importance 
when  the  sensitiveness  of  the  retina  of  different  animals  is  con- 
sidered. Since  these  sensitive  elements  always  lie  as  closely 
together  as  possible,  the  animals  in  which  their  diameter  is 
small  would  have  more  per  given  area,  hence  a  more  sensitive 
retina. 

Another  important  difference  is  the  relative  number  of  rods 
and  cones.  In  mammals  and  amphibians  the  rods  far  surpass 
the  cones  in  number.  In  birds  the  reverse  is  true,  while  in 
reptiles  few  or  no  rods  are  found.  In  fishes  the  rods  and  cones 
are  more  equally  divided.  A  few  exceptions  to  this  are  of 
great  importance  in  substantiating  the  theories  of  the  functions 
of  the  rods  and  cones.  It  has  been  stated  (45)  that  in  the  bat 
and  mole  there  are  no  cones  in  the  yellow  spot  and  in  the 
rabbit  only  a  few.  The  same  is  true  of  other  nocturnal  mam- 
mals which  I  have  examined.  I  have  not  been  able  to  demon- 
strate the  presence  of  cones  in  the  mink,  skunk,  or  rat,  while 
they  are  present  in  the  squirrels.  In  the  night  birds  and  in 
the  eel  very  few  or  no  cones  have  been  demonstrated.  This 
accords  completely  with  the  theory  that  the  rods  function  in 


\ 


No.  3.]  ACUTE    VISION  IN  VERTEBRATES.  ,       487 

the  perception  of  uncolored  and  diffuse  light.  Since  all  colors 
appear  as  gray  by  diffuse  light,  even  though  perceived  by  the 
cones,  and  since  the  rods  are  more  sensitive  to  faint  impressions 
than  the  cones,  the  presence  of  rods  and  almost  complete 
absence  of  cones  in  night  animals  is  no  more  than  can  be 
expected.  Again,  since  the  perception  of  color  is  one  of  the 
important  functions  in  day  animals,  and  as  this  is  done  only  by 
the  cones,  the  relatively  greater  number  of  cones  in  these 
animals  is  readily  accounted  for. 

Acute  vision,  however,  seems  to  depend  on  the  presence  of 
a  fovea.  In  man  the  power  to  see  distinctly  grows  rapidly  less 
from  the  fovea  to  the  ora  serrata.  The  macula,  it  is  true,  sees 
objects  more  distinctly  than  the  peripheral  parts  of  the  retina, 
but  even  this  functions  with  the  peripheral  part  more  as  a  sen- 
tinel for  moving  objects  than  as  a  point  of  acute  vision.  It  is 
true  that  all  animals  are  attracted  more  quickly  by  moving 
objects  than  by  stationary  ones,  and  it  is  especially  true  in  those 
animals  whose  retinal  development  has  not  proceeded  beyond 
the  differentiation  of  an  area.  The  power  of  quiet  and  close 
discrimination  of  objects  at  rest  seems  to  be  present  only  with 
those  animals  which  possess  a  fovea. 

Fishes  as  a  rule  depend  upon  sight  for  their  food,  excepting 
such  as  the  shark,  which  depends  almost  wholly  on  its  smell. 
This  class  of  vertebrates  does  not,  however,  usually  possess  a 
fovea.  How  distinctly  they  see  we  cannot  say,  but  we  know 
that  the  trout  quickly  takes  the  fly  when  thrown  on  the  water, 
or  the  pickerel  the  whirling  spoon  as  it  is  drawn  before  it. 
They  see  the  objects  while  in  motion,  and  are  apparently  unable 
to  distinguish  them  from  the  real  article  of  food.  An  experi- 
ence in  fishing  confirms  the  fact  that  a  pickerel  will  not  bite  at 
a  motionless  spoon-hook.  The  retina  of  these  fish  has  simply 
a  thickening  or  area  at  the  axis  of  vision. 

A  somewhat  similar  experiment  can  be  tried  with  the  frog  or 
toad.  If  one  attaches  a  bit  of  red  flannel,  a  green  leaf,  or  any 
other  small  object  to  a  thread  and  dangles  it  before  a  hungry 
frog,  he  will  quickly  jump  for  it.  A  toad  may  be  fed  on  meat 
in  a  similar  way,  but  in  no  case  will  the  meat  be  taken  unless  it 
is  in  motion.     Neither  do  these  animals  show  any  marked  power 


488  SLONAKER.  [Vol.  XIII. 

of  discrimination  by  sight.  They  will  jump  at  any  small 
moving  object,  and  are  apparently  not  able  to  distinguish  till 
they  have  it  in  their  mouths  whether  it  is  an  article  of  food  or 
a  pebble.  Investigations  again  show  the  presence  of  an  area 
and  absence  of  a  fovea. 

In  some  of  the  reptiles,  however,  a  marked  difference  in 
power  of  discrimination  by  sight  is  noticed.  Experiments  were 
made  wholly  on  a  small  lizard  (horned  toad).  If  a  dead  fly 
were  put  before  him  when  he  was  hungry  he  would  eye  it 
closely  for  a  brief  time,  then  quickly  take  it.  His  aim  was 
always  certain,  never  missing  his  mark,  while  that  of  the  ordi- 
nary toad  was  more  at  random,  throwing  out  her  tongue  indis- 
criminately at  moving  objects.  It  is  true  the  lizard  was 
attracted  more  by  a  live  and  moving  fly  than  by  a  dead,  motion- 
less one,  but  he  also  had  the  power  of  perceiving  things  at 
rest.  This  little  creature  possessed  a  sharp  and  well-defined 
fovea. 

In  general,  birds'  eyes  are  almost  as  perfect  as  man's,  and 
likewise  the  optic  lobes  are  even  greater  in  proportion  to  the 
size  of  the  body  than  that  of  man.  It  is  true  that  the  bird 
often  catches  flies  as  they  buzz  about,  but  it  also  inspects  each 
leaf  carefully  above  and  below  for  a  worm  or  bug  which  may 
be  there  in  hiding,  and  which  it  seldom  fails  to  recognize.  The 
hawk  as  it  soars  high  in  the  heavens  sees  the  snake,  rat,  or 
mouse  in  the  grass,  and  is  frequently  seen  to  dart  and  secure 
its  prey.  Very  acute  sight  is  present  in  all  birds  and  especially 
in  birds  of  prey. 

A  great  difference  exists  in  the  power  of  sight  in  mammals. 
The  primates  possess  the  power  of  most  acute  vision.  Many 
of  the  mammals  depend  on  smell  and  hearing  more  than  on 
sight.  The  dog  picks  his  master  out  of  the  crowd  by  smell;  so 
does  the  sheep  her  lamb.  Sight  in  these  cases  being  only  par- 
tial recognition,  as  they  are  not  sure  until  they  have  confirmed 
their  sight  by  the  sense  of  smell.  The  same  is  true  of  the 
cow,  for  she  must  smell  of  the  strange  cow  when  introduced  into 
the  herd.  The  horse  is  cured  of  his  fright  by  smelling  of  the 
object  which  caused  it.  In  all  these  cases  we  have  a  motion  of 
the  ears,  showing  that  the  animal  is  not  only  using  sight  and 


No.  3-]  ACUTE   VISION  IN  VERTEBRATES.  489 

smell,  but  also  hearing.  Mammals  in  general  do  not  see  a  man 
if  he  remains  quiet,  but  the  crow  easily  recognizes  him,  and 
can  distinguish  his  stick  from  a  gun.  The  dog  looks  into  your 
face,  but  you  cannot  tell  whether  he  is  looking  into  your  eyes 
or  at  your  mouth.  He  has  an  indefinite  gaze,  and,  like  most 
mammals,  is  not  satisfied  with  the  sense  of  sight  alone,  but 
must  confirm  and  improve  by  the  sense  of  smell  and  hearing. 

In  the  present  study  it  is  impossible  to  make  a  more  definite 
comparison  of  the  powers  of  \'ision  in  the  different  vertebrates. 
Many  years  of  careful  observation  of  the  visual  habits  and 
related  histological  structure  of  each  animal  will  be  necessary. 
But  so  far  as  experiments  have  gone,  the  power  of  quiet  and 
close  discrimination  of  an  object  at  rest  seems  to  be  present 
only  in  those  animals  whose  development  of  the  retina  has  pro- 
ceeded a  stage  farther  above  that  of  the  simple  area  —  to  the 
fovea  centralis. 


I. 

1851. 

2. 

1798. 

3- 

1796. 

4- 

1796, 

490  SLONAKER.  [Vol.  XIII. 


REFERENCES. 

Arnold's   Handbuch   der  Anatomie   des   Menschen,  Bd.  ii,  2 

Abtheil.,  pp.  1 038-1 040. 
SOEMMERRING,  Sm.  Th.  V.    Commentat.  soc.  reg.  scient.  Getting., 

Bd.  xiii,  pp.  3-13. — Journ.  der  Erfindungeji,  Theorien  u. 

Widerspruche  in  der  Medicin,  Bd.  xiv,  1 796. 
Buzzi.    Journ.  der  Erfindungen.,  Theorien  u.  Widerspriiche 

in  der  Medicin,  Bd.  xiv,  p.  1 20. 
MiCHAELis,  P.     Ueber  einen  gelben  Fleck  und  ein  Loch  in 

der  Nervenhaut  des  menschlichen  Auges.    Journ.  der  Er- 

Jindungen,  Theorien  u.  Widerspriiche  in  der  Natur-  und 

Arzneiwissenschajt,  Bd.  xv,  pp.  3-17. 

5.  1797.    Reil,  J.  C.     Die  Falte,  der  gelbe  Fleck  und 'die  durchsichtige 

S telle  in  der  Netzhaut  des  Auges.  ReiVs  Arch.f.  Physiol., 
Bd.  ii,  pp.  468-473. 

6.  1797.    Meckel.     ReiVs  Arch.f.  Physiol.,  Bd.  ii,  p.  471. 

7.  1798.    Home,  E.     An  account  of  the  orifice  in  the  human  retina  dis- 

covered by  Professor  Soemmerring,  to  which  are  added 
proofs  of  this  appearance  being  extended  to  the  eyes  of 
other  animals.     Phil.  Trans.,  London,  pp.  332-345. 

8.  1809.    CuviER.     Lemons  d'anatomie  comparde,  tome  ii,  p.  413.  —  Vor- 

lesungen  iiber  vergleichende  Anat.,  Uebersetzt  von  Meckel, 
Bd.  ii,  pp.  410-414. 

9.  1830.    V.  Ammon.     Degenesi   et   usu   macula   luteae  in  retina  oculi 

humani  obviae.     v.  Amtnon's  Zeitschr.,  Bd.  i,  pp.  114,  115. 

10.  1808.    Albers,  J.  F.     Bemerkungen  iiber  den  Bau  der  Augen  ver- 

schiedener  Thiere.  Denkschr.  der  Kon.  Acad,  der  Wiss. 
Miinchen,  pp.  81-90. 

11.  1823.    Knox,  R.     On  the  Discovery  of  the  Foramen  Centrale  of  the 

Retina  in  the  Eyes  of  Reptiles.  The  Edinburgh  Philos. 
Journ.,  vol.  ix,  pp.  358,  359. 

12.  1826.    MuLLER,  JOH.    Zur  Vergl.  Physiol,  des  Gesichtssinnes,  Leipzig, 

p.  102. 

13.  1872.    MuLLER,  H.     Ueber  das  ausgedehnte  Vorkommen  einer  dem 

gelben  Fleck  der  Retina  entsprechenden  Stelle  bei  Thieren. 
Miiller''s  Anatomie  und  Physiologic  des  Auges.  Zusammen- 
gestellt  von  Otto  Becker,  Leipzig,  p.  138. 

14.  1867.    HuLKE,  J.  W.     On  the   Retina   of   Amphibia   and   Reptilia. 

Jour7i.  of  Anat.  and  Physiol.,  vol.  i,  pp.  94-106. 

15.  1868.    Gulliver,  G.    Fovea  Centrahs  in  the  Eye  of  the  Fish.  Journ. 

of  Anat.  and  Physiol.,  vol.  ii,  p.  12. 

16.  1885.    Carriere,    J.      Die    Sehorgane    der   Thiere,    Miinchen    und 

Leipzig,  p.  57. 


No.  3-]  ACUTE    VISION  IN  VERTEBRATES.  491 

17.  Hoffmann.    On  the  Eye  of  the  Crocodile.    Bronn-Klassen  und 

Ordnungen  des  Thierreiches,  Bd.  vi,  erste  Abtheil.,  p.  819. 

18.  1891.     Krause,  W.     Die  Retina.    Internat.  Monatsschrift  fiir  Anat. 

u.  Physiol.,  Bd.  viii,  pp.  414,  415. 

19.  1882.    Ganser.     Zur  Anatomie  der  Katzenretina.    Zeitschrift  f.  ver- 

gleichende  Augenheilkujide,  Heft  2,  pp.  139,  140. 

20.  1856.    MtJLLER,  H.    Anatomisch-physiologische  Untersuchungen  iiber 

die  Retina  des  Menschen  und  der  Wirbelthiere.  Anat.  u. 
Physiol,  des  Aiiges,  pp.  52-134. 

21.  1887.    Tartuferi,  F.     Suir  anatomia  della  retina.     Intertiat.  Zeit- 

schrift f.  Anat.  n.  Physiol,  v.  Krause,  Bd.  iv,  pp.  421-441. 

22.  1 888.    DoGiEL,  A.     Ueber  das  Verhalten  der  nervosen  Elemente  in 

der  Retina  der  Ganoiden,  Reptilien,  Vogel,  u.  Saugethiere. 
Anat.  Anzeiger,  pp.  133-144.  —  Arch./.  mik.Anat.,  Bd.  xU, 
pp.  62-87. 

23.  1890.    Baquis,  E.     La  Retina  della  Faina.    Anat.  Anzeiger,  Nos.  13 

und  14,  pp.  366-371. 

24.  1893.    Ramon  Y  Cajal.     Neue  Darstellung  vom  histologischen  Bau 

des  Centralnervensystems.  Arch.  f.  Anat.  u.  Physiol., 
Anatomische  Abtheilung,  Heft  v  und  vi,  pp.  399-410.  — 
Sur  la  morphologic  et  les  connexions  des  ^Idments  de  la 
rdtine  des  oiseaux.  Anat.  Anzeiger,  No.  4,  pp.  111-121, 
1889. 

25.  1 861.    MtJLLER,  H.      Notiz   iiber   die    Netzhautgefasse   bei   einigen 

Thieren.    Anat.  icnd  Physiol,  des  Attges,  pp.  137,  138,  141. 

26.  HuscHKE.     Eingeweidelehre,  S.  748  und  749.     (H.  Miiller.) 

27.  1890.     Chievitz,  J.  H.    Ueber  die  Entwickelung  der  Area  und  Fovea 

centralis  retina.  Arch.  f.  Anat.  n.  Entwick.,  Leipzig,  Heft 
V,  vi,  pp.  232-366. 

28.  1894.    Ramon  y  Cajal.     Retina  der  Wirbelthieren.    Uebersetzt  und 

herausgegeben  von  Richard  Greeff,   {a)  pp.  149-154;    {b') 

pp.  6-14. 
Dimmer,  F.     Beitrage  zur  Anat.  und  Physiol,  der  Macula  lutea 

des  Menschen,  Leipzig  und  Wien,  p.  54. 
Borysiekiewicz,    M.      Weitere    Untersuchungen    iiber    den 

feineren  Bau  der  Netzhaut,  Leipzig  und  Wien,  p.  56. 
Chievitz,  J.  H.     Ueber  das  Vorkommen  der  Area  centralis 

retinae  in  den  vier  hoheren  Wirbelthierklassen.     Arch.  f. 

Attat.  n.  Entwickelungsgeschichte,  Leipzig,  Heft  iv,  v,  und 

vi,  {a)  pp.  322-324  ;  {b)  p.  327  ;  {c)  p.  326. 

32.  1880.    Denissenko,  G.     Mittheilung  iiber  die  Gefasse  der  Netzhaut 

der  Fische.  Arch.  f.  mikr.  Anat.,  Bd.  x\-iii,  pp.  480- 
485. 

33.  1887.    Borysiekiewicz,  M.     Untersuchungen  iiber  den  feineren  Bau 

der  Netzhaut,  Leipzig  und  Wien,  p.  70. 


29. 

1894, 

30. 

1894, 

31- 

1891. 

492  SLONAKER.  [Vol.  XIII. 

34.  1 81 8.    SOEMMERRING,  D.  W.     De  oculorum  hominis  animalium  que 

sectione  horizontali.    Commentatio.    Gottingen.    (Chievitz.) 

35.  1805.    Blumenbach,  J.  F.     Handbuch  der  Vergl.  Anat,  Gottingen, 

p.  547. 
SCHWALBE.     Lehrbuch  Anatomie  der  Sinnesorgane,  p.  90. 
Krause,  W.     Die  Retina.     Internat.  Monatsschrift  f.  Anat. 

u.  Physiol.,  Bd.  vi. 
SCHIEFFERDECKER.     Anat.  Anzeiger,  No.  12. 
Franklin,  Mrs.  C.  Ladd.     Eine  neue  Theorie  der  Licht- 

empfindungen.    Zeitschr.  f.  Psychologic  u.  Physiologic  der 

Sinnesorgane,  iv. 
Franklin,  Mrs.  C.  Ladd.     On  Theories  of  Light-Sensation. 

Mind,  N.  S.,  2,  pp.  473-489 ;  («)  P-  484  ;  (^)  P-  477- 
Franklin,  Mrs.  C.  Ladd.     Psychological  Review,  vol.  iii, 

No.  2,  p.  230. 
Franklin,  Mrs.  C.   Ladd.     The  Normal  Defect  of  Vision 

in  the  Fovea.     Psychological  Review,  vol.  ii,  p.  143. 
Parinaud,  M.  H.     La  sensibility  de  I'ceil  aux  couleurs  spec- 

trales.     Revtie  Scientifique,  Ser.  4,  tome  iv,  pp.  134-141, 

Aug.  3. 
KoNiG,  Dr.  Arthur.    Sitzungsberichte  der  Koniglichen  Preus- 

sischen  Akademie  der  Wissenschaften  zu  Berlin,  xxx. 
Stewart.     Manual  of  Physiology,  p.  726. 


36. 

1887. 

37- 

1889. 

38. 

1887. 

39- 

40. 

1893. 

41. 

1896. 

42. 

1895. 

43- 

1895. 

44. 

1894. 

45- 

1895. 

No.  3-]  ACUTE    VISION  IN  VERTEBRATES.  493 


EXPLANATION    OF   PLATE   XXVII. 

Fig.  I.  Human,  left  eye,  ^,  showing  nerve  entrance  {N),  blood-vessels,  and 
macula  and  fovea  (^  F). 

Fig.  2.     Gorilla,  left  eye,  \.     N,  nerve  entrance;  A  F,  area  and  fovea. 

Fig.  3.  Dog  (Canis  familiaris),  left  eye,  \.  Shows  nerve  entrance  {N)  and 
blood-vessels  which  were  injected. 

Fig.  4.  Cat  (Felis  catus  domesticus),  left  eye,  \.  N,  nerve  entrance  ;  Ab, 
white,  band-like  region,  which  appears  as  an  area.  The  blood-vessels  were  in- 
jected. 

Fig.  5.  Fox  (Vulpes  vulpes),  left  eye,  \.  N,  nerve  entrance  ;  Ab,  band-like 
area.     Blood-vessels  were  not  injected. 

Fig.  6.  Cow  (Bos  taurus  domesticus),  left  eye,  \.  N,  nerve  entrance  ;  Ab, 
band-like  area. 

Fig.  7.  Horse  (Equus  caballus),  left  eye,  \.  7V^,  nerve  entrance  ;  Ab,  band- 
like area. 

Fig.  8.  Sheep  (Ovis  avies),  left  eye,  \.  N,  nerve  entrance  ;  Ab,  band-like 
area.     Blood-vessels  were  injected. 

Fig.  9.  Pig  (Sus  domesticus),  left  eye,  \.  N,  nerve  entrance ;  Ab,  band-like 
area. 

Fig.  10.  Barred  Owl  (Syrnium  nebulosum),  left  eye,  \.  N,  nerve  entrance  ; 
P,  pecten ;  A  F,  area  and  fovea. 

Fig.  II.  Red-Tailed  Buzzard  (Buteo  borealis),  left  eye,  \.  N,  nerve  entrance; 
F,  pecten;  Ft,  At,  fovea  and  area  temporalis;  Fn,  An,  fovea  and  area  nasalis; 
Ab,  band-like  area. 

Fig.  12.  Tern  (Sterna  hirundo),  left  eye,  \.  TV,  nerve  entrance  ;  P,  pecten  ; 
Ft,  At,  fovea  and  area  temporalis  ;  Fn,  Ati,  fovea  and  area  nasalis  ;  Ab,  band-like 
area.  A  dark  line  extending  along  the  band-like  area  corresponds  to  Chievitz's 
trough-like  fovea.     In  cross  sections  no  such  fovea  is  found. 

Fig.  13.  Rabbit  (Lepus  sylvaticus),  left  eye,  \.  A'',  nerve  entrance;  Ab,  band- 
like area.     The  blood-vessels  were  injected. 

Fig.  14.  Fox  Squirrel  (Sciurus  niger),  left  eye,  ^.  N,  nerve  entrance;  Ab, 
area  not  visible  to  the  naked  eye. 

Fig.  15.  Homed  Toad  (Phrynosoma  cornutum),  left  eye,  \.  N,  nerve  en- 
trance ;  P,  conical  pecten;  F,  fovea;  Ab,  band-like  area. 

Fig.  16.  Turtle  (Chelopus  insculptus),  left  eye,  \.  N,  nerve  entrance  ;  A, 
area,  not  \isible  to  the  naked  eye ;  Bv,  an  apparent  rudimentary  blood-vessel 
which  was  injected. 

Fig.  17.  Robin  (Merula  migratoria),  left  eye,  \.  N,  nerve  entrance;  P, 
pecten  ;  A  F,  area  and  fovea. 

Fig.  18.  Frog  (Rana  catesbiana),  left  eye, -J^.  TV,  nerve  entrance;  /4(5,  band- 
like area,  not  visible  to  the  naked  eye. 

Fig.  19.  Sparrow  Hawk  (Falco  sparverius),  left  eye,  |.  A^,  nerve  entrance ; 
P,  pecten ;  An,  Fn,  area  and  fovea  nasalis  ;  At,  Ft,  area  and  fovea  temporalis  ; 
Ab,  band-like  area. 

Fig.  20.  Ring-Neck  Plover  (^Egialitis  semipalmata),  left  eye,  \.  N,  nerve  en- 
trance ;  P,  pecten  ;  A  F,  area  and  fovea  ;  Ab,  band-like  area. 


494 


SLONAKER. 


Fig.  21.  Chicken  (Gallus  domesticus),  left  eye,  \.  N,  nerve  entrance  ;  P, 
pecten. 

Fig.  22.  Diagrammatic  representation  of  the  comparative  thickness  of  the  layers 
of  the  retina  in  the  different  vertebrates.  Measurements  were  taken  of  the  retina 
at  corresponding  positions  and  magnified  130  diameters,  i.  Nerve-fibre  layer; 
2.  Nerve-cell  layer  ;  3.  Inner  molecular  layer ;  4.  Inner  nuclear  layer ;  5.  Outer 
molecular  layer  ;  6.  Outer  nuclear  layer ;  7.  Rod  and  cone  layer ;  8.  Pigment 
layer.  The  last  two  layers  generally  overlap.  I.  Human.  II.  Cat  (Felis  catus 
domesticus).  III.  Blue  Jay  (Cyanocitta  cristata).  IV.  Snake  (Eutainia  sirtalis). 
V.  Frog  (Rana  catesbiana).     VI.  Pickerel  (Esox  reticulatus) . 

Fig.  23.  Black-Crowned  Night  Heron  (Nycticorax  nycticorax),  left  eye,  \. 
N,  nerve  entrance ;  F,  pecten  ;  AF,  area  and  fovea. 


lournal  of  Morphology         Plate  XXVII 


First  /fa  If 


a' 


-ft 


-T 


-t* 


Atel- 


Fig.   1.  Fig.  2.  I'ig-   3.  ^^S.   4. 


Fig.    11. 


Fig.    12. 


\ 


Journal  vf  Morphofogt/       P/ale  XYl/JI 


Second  Half 


.flb 


Fig,   13. 


Fig.   18. 


Fig.  21. 

J 

IL 

m. 
Fig.  22.     jjL 


Fig.   19. 


^r 


/  2   3       f  S      6  7    y 


/  X3     -^    6'      6             7        V 

H 


U-Vw,  a  w  . 


/     ,ir, 2_ 


6-6      7         Y 


3      _      ■¥       f-f, 


.'■'^.'^'■i 


/  2       J  -(^    ^  tf       7  V     f 


A  vv\\3V\  vh  I  0. 


v-v 


Fig.  23. 


ACUTE    VISIOX  IN  VERTEBRATES.  495 


EXPLANATION   OF    PLATE     XXVIIL 

Fig.  24.  Human,  age  4.  Horizontal  section  through  the  center  of  fovea  of 
the  right  eye.  The  eye  was  enucleated  during  life  and  immersed  at  once  in  the 
hardening  fluid.     Section  36^  thick.     X  32.3. 

Fig.  25.  Human,  adult.  Horizontal  section  of  right  eye  through  center  of 
fovea.  Eye  was  enucleated  during  life  and  subjected  at  once  to  the  hardening 
fluids.     Section  36^11  thick.     X  32.3. 

Fig.  26.  Gorilla.  Horizontal  section  of  right  eye  through  center  of  fovea. 
The  eye  was  about  nine  hours /^j^  mortem  and  the  apparent  depth  of  fovea  is  due 
to  the  folds  of  the  retina  in  the  macula.     Section  36/x  thick.     X  32.3. 

Fig.  27.  Gorilla.  Vertical  section  through  the  center  of  the  fovea  and  the 
folds  of  the  macula,  showing  the  folds  as  they  appear  due  to  post  mortevi  changes. 
Section  36M  thick.     X  32.3. 

Fig.  28.  Robin  (Merula  migratoria).  Horizontal  section  through  center  of  the 
fovea  of  right  eye.  Eye  was  subjected  to  hardening  fluids  immediately  after  death. 
Section  24M  thick.     X  32.3. 

Fig.  29.  Blue-Bird  (Sialia  sialis).  Horizontal  section  through  center  of  the 
fovea  of  the  left  eye.  Subjected  to  hardening  fluids  immediately  after  death. 
Section  iSjot  thick.     X  32.3. 

Fig.  30.  Kinglet  (Regulus  satrapa).  Horizontal  section  through  center  of 
the  fovea  of  the  right  eye.  The  head  was  subjected  at  once  after  death  to  hard- 
ening fluids  and  sections  afterwards  made  through  whole  head  mth  eyes  in  situ. 
This  section  includes  not  only  the  bottom  of  the  fovea,  but  also  some  of  the  cells 
of  the  area  beyond.     Section  24^  thick.     X   32.3. 

Fig.  31.  Snow-Bird  (Junco  hyemalis).  Horizontal  section  through  center  of 
the  fovea  of  right  eye.  This  eye  was  hardened  by  the  injection  method.  The 
retina  in  the  region  of  the  fovea  floated  off  from  the  choroid.  Section  36/x 
thick.     X  32.3. 

Fig.  32.  Goose  (Anser  cinereus  domesticus).  Vertical  section  of  right  eye. 
Across  band-like  area  on  the  nasal  side  of  the  fovea  about  midway  to  the  ora 
serrata.  The  arrow  points  to  the  center  of  the  area.  Eye  was  subjected  to  hard- 
ening fluids  immediately  after  death.     Section  36,u  thick.     X  32.3. 

Fig.  33.  Goose.  Vertical  section  through  the  center  of  the  fovea  of  right 
eye.     Section  36^1  thick.     X  32.3. 

Fig.  34.  Goose.  Vertical  section  of  right  eye  across  band-like  area  on  the 
temporal  side  of  fovea  about  midway  to  ora  serrata.  The  arrow  points  to  the 
center  of  the  area.  A  fold  in  the  section  partiy  obscures  the  area.  Section  36^ 
thick.     X  32.3. 

Fig.  35.  Turkey  (Meleagris  gallopavo).  Horizontal  section  through  center 
of  the  fovea  of  right  eye.  Eye  was  immersed  at  once  after  death  in  hardening 
fluid.     Section  36^  thick.     X  32.3. 

Fig.  36.  Guinea  Hen  (Numida  pucherani).  Horizontal  section  through  center 
of  area  of  right  eye.  The  eye  was  subjected  to  hardening  fluids  at  once  after 
death.  The  arrow  points  to  the  center  of  the  area  where  a  very  slight  pitting  may 
be  seen,  which  may  possibly  be  called  a  fovea.     Section  24^  thick.     X  32.3. 


496  SLONAKER. 

Fig.  37.  Pigeon  (Columba  livia  domestica).  Adult.  Horizontal  section 
through  center  of  the  fovea  of  right  eye.     Section  i8/U  thick.      X  32.3. 

Fig.  38.  Pigeon.  Horizontal  section  through  center  of  fovea  of  right  eye. 
Section  24,14  thick.     X  32.3. 

Fig.  39.  Surf  Duck  (Oidema  deglandi).  Horizontal  section  through  center 
of  the  fovea  of  left  eye.  Eye  about  three  hours  post  mortem.  Section  30/t  thick. 
X  32-3- 


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498  SLONAKER. 


EXPLANATION   OF   PLATE   XXIX. 

.  Fig.  40.  Tern  (Sterna  hirundo).  Vertical  section  across  band-like  area  (a)  on 
the  nasalis  side  of  the  fovea  nasalis.     Section  36/*  thick.     X  32.3. 

Fig.  41.  Tern  (Sterna  hirundo).  Horizontal  section  through  center  of  fovea 
nasalis.     Section  30^  thick.     X  32.3. 

Fig.  42.  Tern  (Sterna  hirundo).  Vertical  section  across  band-like  area  {a) 
about  midway  between  fovea  nasalis  and  temporalis.     Section  36/a  thick.     X  32.3. 

Fig.  43.  Tern  (Sterna  hirundo).  Horizontal  section  through  center  of  fovea 
temporalis.     Section  30/i  thick.     X  32.3. 

Fig.  44.  Kingfisher  (Ceryle  alcyon).  Horizontal  section  through  center  of 
fovea  nasalis.     Section  30/t  thick.     X  32.3. 

Fig.  45.  Kingfisher  (Ceryle  alcyon).  Horizontal  section  through  center  of 
fovea  temporalis.     Section  30/i  thick.     32.3. 

Fig.  46.  White-Bellied  Swallow  (Tachycineta  bicolor).  Horizontal  section 
through  center  of  fovea  nasalis.     Section  24/11  thick.     X  32.3. 

Fig.  47.  Ring-Neck  Plover  (^gialitis  semipalmata).  Horizontal  section 
through  fovea  of  left  eye.     Section  i8,u  thick.     X  32.3. 

Figs.  48,  49.  Sparrow  Hawk  (Falco  sparverius).  Section  passed  through 
each  fovea  and  denter  of  pupil.  Fig.  48,  fovea  nasalis  and  Fig.  49,  fovea  tempo- 
ralis.    Section  42/^  thick.     X  32.3. 

Figs.  50,  51.  Red-Tailed  Buzzard  (Buteo  borealis).  Section  passed  in  plane 
of  both  foveas  and  center  of  pupil.  Fig.  50,  fovea  nasalis  and  Fig.  51,  fovea 
temporalis.     Section  42jU  thick.     X  32.3. 

Fig.  52.  Crow  (Corvus  americanus).  Horizontal  section  through  center  of 
fovea.     Section  36/x  thick.     X    32.3. 

Fig.  53.  Blue  Jay  (Cyanocitta  cristata).  Horizontal  section  through  center 
of  the  fovea.     Section  48/x  thick.     X  32.3. 

Fig.  54.  Shorepeep  (Ereunetes  pusillus).  Horizontal  section  through  center 
of  fovea  of  left  eye.  Shows  some  cells  of  the  area  lying  beyond  the  center  of  the 
fovea.     Section  24/^1  thick.     X  32.3. 

Fig.  55.  Barred  Owl  (Syrnium  nebulosum).  Horizontal  section  of  right  eye 
through  center  of  fovea  (fovea  temporalis).     Section  48/u  thick.     X  32.3. 


Journal  of  Moi-phology    Vol.  XIII 


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500  SLONAKER. 


EXPLANATION    OF   PLATE   XXX. 

Figs.  56,  57.  Horned  Toad  (Phrynosoma  cornutum).  Fig.  56,  vertical  sec- 
tion and  Fig.  57,  horizontal  section  through  the  center  of  each  fovea.  Fig.  56 
shows  also  the  width  of  the  band-like  area.     Sections  iZfj.  thick.     X  32.3. 

Fig.  58.  Pipefish  (Siphostoma  fuscum).  Horizontal  section  through  center 
of  fovea  (i)  of  left  eye.  2  indicates  the  position  of  nerve  entrance.  Section  i8/x 
thick.     X    32.3. 

Fig.  59.  Pipefish  (Siphostoma  fuscum).  Section  in  a  lower  plane,  showing 
entrance  of  nerve  (2)  and  the  area  (i).     Section  iS/x  thick.     X  32.3. 

Fig.  60.  Chipmunk  (Tamias  striatus).  Vertical  section  across  area  (a). 
Section  24;U  thick.     X  32.3. 

Fig.  61.  Turtle  (Chelydra  serpentina).  Horizontal  section  through  band-like 
area.     Section  i8;ti  thick.     X  32.3. 

Fig.  62.  Frog  (Rana  catesbiana).  Vertical  section  across  band-like  area  (a). 
Section  24^  thick.     X  32.3. 

Fig.  63.  Flounder  (Paralichthys  dentatus).  Vertical  section,  showing  com- 
parative thickness  of  different  layers  of  the  retina.     Section  30^14  thick.     X  32.3. 


vToui'inil  oi' Mor'phohujij     f()/.  XIII 


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Plate  XXX 


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Fiy.ea 


502  ,  SLONAKER. 


EXPLANATION   OF   PLATE   XXXL 

The  following  figures  were  made  from  sections  through  the  whole  head  with 
eyes  in  situ.  i^A^  marks  the  axis  of  vision  for  the  fovea  nasalip  ;  FT,  fovea  tem- 
poralis ;  and  Op,  the  entrance  of  the  optic  nerve  indicated  by  the  pecten. 

The  slight  divergence  of  the  lines  of  binocular  vision  {FT)  is  probably  due  to 
the  relaxation  of  the  internal  recti  muscles  after  death. 

Fig.  64.     White-Bellied  Swallow  (Tachycineta  bicolor),  \. 

Fig.  65.     Common  Tern  (Sterna  hirundo),  \. 

Fig.  66.     Sparrow  Hawk  (Falco  sparverius),  \. 

Fig.  67.     Broad- Winged  Hawk  (Buteo  latissimus),  \. 

Fig.  68.     Great  Horned  Owl  (Bubo  virginianus),  \. 


Joarnal  of  Morphologfi/ 


P/ate    XXXI 


Fig-.   64, 


Fig.  6  5. 


Fig.  67. 


Fig.  66 


Fig-.   68 


Date  Due 


i 


SL5 
QP479 

jof  acute  vision  a^* 


